Icnirp Guidelines Guidelines For Limiting Exposure To Time Varying ...
ICNIRP Guidelines
GUIDELINES FOR LIMITING EXPOSURE TO TIME-VARYING
ELECTRIC, MAGNETIC, AND ELECTROMAGNETIC FIELDS
(UP TO 300 GHz)
International Commission on Non-Ionizing Radiation Protection*†
INTRODUCTION
At the Eighth International Congress of the IRPA
(Montreal, 18 –22 May 1992), a new, independent scien-
IN 1974, the International Radiation Protection Associa-
tific organization—the International Commission on
tion (IRPA) formed a working group on non-ionizing
Non-Ionizing Radiation Protection (ICNIRP)—was es-
radiation (NIR), which examined the problems arising in
tablished as a successor to the IRPA/INIRC. The func-
the field of protection against the various types of NIR.
tions of the Commission are to investigate the hazards
At the IRPA Congress in Paris in 1977, this working
that may be associated with the different forms of NIR,
group became the International Non-Ionizing Radiation
develop international guidelines on NIR exposure limits,
and deal with all aspects of NIR protection.
Committee (INIRC).
Biological effects reported as resulting from expo-
In cooperation with the Environmental Health Divi-
sure to static and extremely-low-frequency (ELF) elec-
sion of the World Health Organization (WHO), the
tric and magnetic fields have been reviewed by UNEP/
IRPA/INIRC developed a number of health criteria
WHO/IRPA (1984, 1987). Those publications and a
documents on NIR as part of WHO’s Environmental
number of others, including UNEP/WHO/IRPA (1993)
Health Criteria Programme, sponsored by the United
and Allen et al. (1991), provided the scientific rationale
Nations Environment Programme (UNEP). Each docu-
for these guidelines.
ment includes an overview of the physical characteris-
A glossary of terms appears in the Appendix.
tics, measurement and instrumentation, sources, and
applications of NIR, a thorough review of the literature
on biological effects, and an evaluation of the health risks
PURPOSE AND SCOPE
of exposure to NIR. These health criteria have provided
the scientific database for the subsequent development of
The main objective of this publication is to establish
guidelines for limiting EMF exposure that will provide
exposure limits and codes of practice relating to NIR.
protection against known adverse health effects. An
adverse health effect causes detectable impairment of the
health of the exposed individual or of his or her off-
spring; a biological effect, on the other hand, may or may
* ICNIRP Secretariat, c/o Dipl.-Ing. Ru¨diger Matthes, Bundesamt
not result in an adverse health effect.
fu¨r Strahlenschutz, Institut fu¨r Strahlenhygiene, Ingolsta¨dter Land-
strasse 1, D-85764 Oberschleissheim, Germany.
Studies on both direct and indirect effects of EMF
† During the preparation of these guidelines, the composition of
are described; direct effects result from direct interaction
the Commission was as follows: A. Ahlbom (Sweden); U. Bergqvist
of fields with the body, indirect effects involve interactions
(Sweden); J. H. Bernhardt, Chairman since May 1996 (Germany); J. P.
with an object at a different electric potential from the body.
Ce´sarini (France); L. A. Court, until May 1996 (France); M. Gran-
dolfo, Vice-Chairman until April 1996 (Italy); M. Hietanen, since May
Results of laboratory and epidemiological studies, basic
1996 (Finland); A. F. McKinlay, Vice-Chairman since May 1996
exposure criteria, and reference levels for practical hazard
(UK); M. H. Repacholi, Chairman until April 1996, Chairman emer-
assessment are discussed, and the guidelines presented
itus since May 1996 (Australia); D. H. Sliney (USA); J. A. J. Stolwijk
apply to occupational and public exposure.
(USA); M. L. Swicord, until May 1996 (USA); L. D. Szabo (Hun-
Guidelines on high-frequency and 50/60 Hz electro-
gary); M. Taki (Japan); T. S. Tenforde (USA); H. P. Jammet (Emeritus
Member, deceased) (France); R. Matthes, Scientific Secretary
magnetic fields were issued by IRPA/INIRC in 1988 and
(Germany).
1990, respectively, but are superseded by the present
During the preparation of this document, ICNIRP was supported
guidelines which cover the entire frequency range of
by the following external experts: S. Allen (UK), J. Brix (Germany),
time-varying EMF (up to 300 GHz). Static magnetic
S. Eggert (Germany), H. Garn (Austria), K. Jokela (Finland), H.
Korniewicz (Poland), G.F. Mariutti (Italy), R. Saunders (UK), S.
fields are covered in the ICNIRP guidelines issued in
Tofani (Italy), P. Vecchia (Italy), E. Vogel (Germany). Many valuable
1994 (ICNIRP 1994).
comments provided by additional international experts are gratefully
In establishing exposure limits, the Commission
acknowledged.
recognizes the need to reconcile a number of differing
(Manuscript received 2 October 1997; accepted 17 November 1997)
expert opinions. The validity of scientific reports has to
0017-9078/98/$3.00/0
Copyright © 1998 Health Physics Society
be considered, and extrapolations from animal experi-
494
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
495
● ICNIRP GUIDELINES
ments to effects on humans have to be made. The
the recommended reference levels. Advice on avoiding
restrictions in these guidelines were based on scientific
these problems is beyond the scope of the present
data alone; currently available knowledge, however,
document but is available elsewhere (UNEP/WHO/IRPA
indicates that these restrictions provide an adequate level
1993).
of protection from exposure to time-varying EMF. Two
These guidelines will be periodically revised and
classes of guidance are presented:
updated as advances are made in identifying the adverse
health effects of time-varying electric, magnetic, and
● Basic restrictions: Restrictions on exposure to
electromagnetic fields.
time-varying electric, magnetic, and electromag-
netic fields that are based directly on established
health effects are termed “basic restrictions.”
QUANTITIES AND UNITS
Depending upon the frequency of the field, the
Whereas electric fields are associated only with the
physical quantities used to specify these restric-
presence of electric charge, magnetic fields are the result
tions are current density (J), specific energy
of the physical movement of electric charge (electric
absorption rate (SAR), and power density (S).
current). An electric field, E, exerts forces on an electric
Only power density in air, outside the body, can
charge and is expressed in volt per meter (V m 1).
be readily measured in exposed individuals.
Similarly, magnetic fields can exert physical forces on
● Reference levels: These levels are provided for
electric charges, but only when such charges are in
practical exposure assessment purposes to deter-
motion. Electric and magnetic fields have both magni-
mine whether the basic restrictions are likely to be
tude and direction (i.e., they are vectors). A magnetic
exceeded. Some reference levels are derived from
field can be specified in two ways—as magnetic flux
relevant basic restrictions using measurement
density, B, expressed in tesla (T), or as magnetic field
and/or computational techniques, and some ad-
strength, H, expressed in ampere per meter (A m 1). The
dress perception and adverse indirect effects of
two quantities are related by the expression:
exposure to EMF. The derived quantities are
electric field strength (E), magnetic field strength
B
H,
(1)
(H), magnetic flux density (B), power density (S),
where
is the constant of proportionality (the magnetic
and currents flowing through the limbs (I ).
L
permeability); in a vacuum and in air, as well as in
Quantities that address perception and other indi-
non-magnetic (including biological) materials,
has the
rect effects are contact current (I ) and, for pulsed
C
value 4
10 7 when expressed in henry per meter
fields, specific energy absorption (SA). In any
(H m 1). Thus, in describing a magnetic field for
particular exposure situation, measured or calcu-
protection purposes, only one of the quantities B or H
lated values of any of these quantities can be
needs to be specified.
compared with the appropriate reference level.
In the far-field region, the plane-wave model is a
Compliance with the reference level will ensure
good approximation of the electromagnetic field propa-
compliance with the relevant basic restriction. If
gation. The characteristics of a plane wave are:
the measured or calculated value exceeds the
● The wave fronts have a planar geometry;
reference level, it does not necessarily follow that
● The E and H vectors and the direction of propa-
the basic restriction will be exceeded. However,
gation are mutually perpendicular;
whenever a reference level is exceeded it is
● The phase of the E and H fields is the same, and
necessary to test compliance with the relevant
the quotient of the amplitude of E/H is constant
basic restriction and to determine whether addi-
throughout space. In free space, the ratio of their
tional protective measures are necessary.
amplitudes E/H
377 ohm, which is the charac-
teristic impedance of free space;
These guidelines do not directly address product
● Power density, S, i.e., the power per unit area
performance standards, which are intended to limit EMF
normal to the direction of propagation, is related
emissions under specified test conditions, nor does the
document deal with the techniques used to measure any
to the electric and magnetic fields by the expression:
of the physical quantities that characterize electric, mag-
S
EH
E2/377
377H2.
(2)
netic, and electromagnetic fields. Comprehensive de-
scriptions of instrumentation and measurement tech-
The situation in the near-field region is rather more
niques
for
accurately
determining
such
physical
complicated because the maxima and minima of E and H
quantities may be found elsewhere (NCRP 1981; IEEE
fields do not occur at the same points along the direction
1992; NCRP 1993; DIN VDE 1995).
of propagation as they do in the far field. In the near field,
Compliance with the present guidelines may not
the electromagnetic field structure may be highly inho-
necessarily preclude interference with, or effects on,
mogeneous, and there may be substantial variations from
medical devices such as metallic prostheses, cardiac
the plane-wave impedance of 377 ohms; that is, there
pacemakers and defibrillators, and cochlear implants.
may be almost pure E fields in some regions and almost
Interference with pacemakers may occur at levels below
pure H fields in others. Exposures in the near field are
496
Health Physics
April 1998, Volume 74, Number 4
Table 1. Electric, magnetic, electromagnetic, and dosimetric
these guidelines are based on short-term, immediate
quantities and corresponding SI units.
health effects such as stimulation of peripheral nerves
Quantity
Symbol
Unit
and muscles, shocks and burns caused by touching
conducting objects, and elevated tissue temperatures
Conductivity
siemens per meter (S m 1)
resulting from absorption of energy during exposure to
Current
I
ampere (A)
Current density
J
ampere per square meter (A m 2)
EMF. In the case of potential long-term effects of
Frequency
f
hertz (Hz)
exposure, such as an increased risk of cancer, ICNIRP
Electric field strength
E
volt per meter (V m 1)
concluded that available data are insufficient to provide a
Magnetic field strength
H
ampere per meter (A m 1)
basis for setting exposure restrictions, although epidemi-
Magnetic flux density
B
tesla (T)
Magnetic permeability
henry per meter (H m 1)
ological research has provided suggestive, but uncon-
Permittivity
farad per meter (F m 1)
vincing, evidence of an association between possible
Power density
S
watt per square meter (W m 2)
carcinogenic effects and exposure at levels of 50/60 Hz
Specific energy absorption
SA
joule per kilogram (J kg 1)
magnetic flux densities substantially lower than those
Specific energy absorption
SAR
watt per kilogram (W kg 1)
recommended in these guidelines.
rate
In-vitro effects of short-term exposure to ELF or
ELF amplitude-modulated EMF are summarized. Tran-
sient cellular and tissue responses to EMF exposure have
more difficult to specify, because both E and H fields
been observed, but with no clear exposure-response
must be measured and because the field patterns are
relationship. These studies are of limited value in the
morecomplicated; in this situation, power density is no
assessment of health effects because many of the re-
longer an appropriate quantity to use in expressing
sponses have not been demonstrated in vivo. Thus,
exposure restrictions (as in the far field).
in-vitro studies alone were not deemed to provide data
Exposure to time-varying EMF results in internal
that could serve as a primary basis for assessing possible
body currents and energy absorption in tissues that
health effects of EMF.
depend on the coupling mechanisms and the frequency
involved. The internal electric field and current density
COUPLING MECHANISMS BETWEEN FIELDS
are related by Ohm’s Law:
AND THE BODY
J
E,
(3)
There are three established basic coupling mecha-
nisms through which time-varying electric and magnetic
where
is the electrical conductivity of the medium. The
fields interact directly with living matter (UNEP/WHO/
dosimetric quantities used in these guidelines, taking into
IRPA 1993):
account different frequency ranges and waveforms, are
● coupling to low-frequency electric fields;
as follows:
● coupling to low-frequency magnetic fields; and
● Current density, J, in the frequency range up to
● absorption of energy from electromagnetic fields.
10 MHz;
● Current, I, in the frequency range up to 110 MHz;
Coupling to low-frequency electric fields
● Specific energy absorption rate, SAR, in the
The interaction of time-varying electric fields with
frequency range 100 kHz–10 GHz;
the human body results in the flow of electric charges
● Specific energy absorption, SA, for pulsed fields
(electric current), the polarization of bound charge (for-
in the frequency range 300 MHz–10 GHz; and
mation of electric dipoles), and the reorientation of
● Power density, S, in the frequency range
electric dipoles already present in tissue. The relative
10 –300 GHz.
magnitudes of these different effects depend on the
electrical properties of the body—that is, electrical con-
A general summary of EMF and dosimetric quanti-
ductivity (governing the flow of electric current) and
ties and units used in these guidelines is provided in
permittivity (governing the magnitude of polarization
Table 1.
effects). Electrical conductivity and permittivity vary
with the type of body tissue and also depend on the
frequency of the applied field. Electric fields external to
BASIS FOR LIMITING EXPOSURE
the body induce a surface charge on the body; this results
These guidelines for limiting exposure have been
in induced currents in the body, the distribution of which
developed following a thorough review of all published
depends on exposure conditions, on the size and shape of
scientific literature. The criteria applied in the course of
the body, and on the body’s position in the field.
the review were designed to evaluate the credibility of
the various reported findings (Repacholi and Stolwijk
Coupling to low-frequency magnetic fields
1991; Repacholi and Cardis 1997); only established
The physical interaction of time-varying magnetic
effects were used as the basis for the proposed exposure
fields with the human body results in induced electric
restrictions. Induction of cancer from long-term EMF
fields and circulating electric currents. The magnitudes
exposure was not considered to be established, and so
of the induced field and the current density are propor-
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
497
● ICNIRP GUIDELINES
tional to the radius of the loop, the electrical conductivity
When the long axis of the human body is parallel to
of the tissue, and the rate of change and magnitude of the
the electric field vector, and under plane-wave exposure
magnetic flux density. For a given magnitude and fre-
conditions (i.e., far-field exposure), whole-body SAR
quency of magnetic field, the strongest electric fields are
reaches maximal values. The amount of energy absorbed
induced where the loop dimensions are greatest. The
depends on a number of factors, including the size of the
exact path and magnitude of the resulting current induced
exposed body. “Standard Reference Man” (ICRP 1994),
in any part of the body will depend on the electrical
if not grounded, has a resonant absorption frequency
conductivity of the tissue.
close to 70 MHz. For taller individuals the resonant
The body is not electrically homogeneous; however,
absorption frequency is somewhat lower, and for shorter
induced current densities can be calculated using ana-
adults, children, babies, and seated individuals it may
tomically and electrically realistic models of the body
exceed 100 MHz. The values of electric field reference
and computational methods, which have a high degree of
levels are based on the frequency-dependence of human
anatomical resolution.
absorption; in grounded individuals, resonant frequencies
are lower by a factor of about 2 (UNEP/WHO/IRPA
1993).
Absorption of energy from electromagnetic fields
For some devices that operate at frequencies above
Exposure to low-frequency electric and magnetic
10 MHz (e.g., dielectric heaters, mobile telephones),
fields normally results in negligible energy absorption
human exposure can occur under near-field conditions.
and no measurable temperature rise in the body. How-
The frequency-dependence of energy absorption under
ever, exposure to electromagnetic fields at frequencies
these conditions is very different from that described for
above about 100 kHz can lead to significant absorption
far-field conditions. Magnetic fields may dominate for
of energy and temperature increases. In general, expo-
certain devices, such as mobile telephones, under certain
sure to a uniform (plane-wave) electromagnetic field
exposure conditions.
results in a highly non-uniform deposition and distribu-
The usefulness of numerical modeling calculations,
tion of energy within the body, which must be assessed
as well as measurements of induced body current and
by dosimetric measurement and calculation.
tissue field strength, for assessment of near-field expo-
As regards absorption of energy by the human body,
sures has been demonstrated for mobile telephones,
electromagnetic fields can be divided into four ranges
walkie-talkies, broadcast towers, shipboard communica-
(Durney et al. 1985):
tion sources, and dielectric heaters (Kuster and Balzano
1992; Dimbylow and Mann 1994; Jokela et al. 1994;
● frequencies from about 100 kHz to less than about
Gandhi 1995; Tofani et al. 1995). The importance of
20 MHz, at which absorption in the trunk de-
these studies lies in their having shown that near-field
creases rapidly with decreasing frequency, and
exposure can result in high local SAR (e.g., in the head,
significant absorption may occur in the neck and
wrists, ankles) and that whole-body and local SAR are
legs;
strongly dependent on the separation distance between
● frequencies in the range from about 20 MHz to
the high-frequency source and the body. Finally, SAR
300 MHz, at which relatively high absorption can
data obtained by measurement are consistent with data
occur in the whole body, and to even higher
obtained from numerical modeling calculations. Whole-
values if partial body (e.g., head) resonances are
body average SAR and local SAR are convenient quan-
considered;
tities for comparing effects observed under various ex-
● frequencies in the range from about 300 MHz to
posure conditions. A detailed discussion of SAR can be
several GHz, at which significant local, non-
found elsewhere (UNEP/WHO/IRPA 1993).
uniform absorption occurs; and
At frequencies greater than about 10 GHz, the depth
● frequencies above about 10 GHz, at which energy
of penetration of the field into tissues is small, and SAR
absorption occurs primarily at the body surface.
is not a good measure for assessing absorbed energy; the
incident power density of the field (in W m 2) is a more
In tissue, SAR is proportional to the square of the
appropriate dosimetric quantity.
internal electric field strength. Average SAR and SAR
distribution can be computed or estimated from labora-
tory measurements. Values of SAR depend on the fol-
INDIRECT COUPLING MECHANISMS
lowing factors:
There are two indirect coupling mechanisms:
● the incident field parameters, i.e., the frequency,
intensity, polarization, and source– object config-
● contact currents that result when the human body
uration (near- or far-field);
comes into contact with an object at a different
● the characteristics of the exposed body, i.e., its
electric potential (i.e., when either the body or the
size and internal and external geometry, and the
object is charged by an EMF); and
dielectric properties of the various tissues; and
● coupling of EMF to medical devices worn by, or
● ground effects and reflector effects of other ob-
implanted in, an individual (not considered in this
jects in the field near the exposed body.
document).
498
Health Physics
April 1998, Volume 74, Number 4
The charging of a conducting object by EMF causes
and VDUs. Most currently available information fails to
electric currents to pass through the human body in
support an association between occupational exposure to
contact with that object (Tenforde and Kaune 1987;
VDUs and harmful reproductive effects (NRPB 1994a;
UNEP/WHO/IRPA 1993). The magnitude and spatial
Tenforde 1996).
distribution of such currents depend on frequency, the
size of the object, the size of the person, and the area of
contact; transient discharges—sparks— can occur when
Residential cancer studies. Considerable contro-
an individual and a conducting object exposed to a strong
versy surrounds the possibility of a link between expo-
field come into close proximity.
sure to ELF magnetic fields and an elevated risk of
cancer. Several reports on this topic have appeared since
BIOLOGICAL BASIS FOR LIMITING
Wertheimer and Leeper reported (1979) an association
EXPOSURE (UP TO 100 KHZ)
between childhood cancer mortality and proximity of
homes to power distribution lines with what the research-
The following paragraphs provide a general review
ers classified as high current configuration. The basic
of relevant literature on the biological and health effects
hypothesis that emerged from the original study was that
of electric and magnetic fields with frequency ranges up
the contribution to the ambient residential 50/60 Hz
to 100 kHz, in which the major mechanism of interaction
magnetic fields from external sources such as power
is induction of currents in tissues. For the frequency
lines could be linked to an increased risk of cancer in
range
0 to 1 Hz, the biological basis for the basic
childhood.
restrictions and reference levels are provided in ICNIRP
To date there have been more than a dozen studies
(1994). More detailed reviews are available elsewhere
on childhood cancer and exposure to power-frequency
(NRPB 1991, 1993; UNEP/WHO/IRPA 1993; Blank
magnetic fields in the home produced by nearby power
1995; NAS 1996; Polk and Postow 1996; Ueno 1996).
lines. These studies estimated the magnetic field expo-
sure from short term measurements or on the basis of
Direct effects of electric and magnetic fields
distance between the home and power line and, in most
cases, the configuration of the line; some studies also
Epidemiological studies. There have been many
took the load of the line into account. The findings
reviews of epidemiological studies of cancer risk in
relating to leukemia are the most consistent. Out of 13
relation to exposure to power-frequency fields (NRPB
studies (Wertheimer and Leeper 1979; Fulton et al. 1980;
1992, 1993, 1994b; ORAU 1992; Savitz 1993; Heath
Myers et al. 1985; Tomenius 1986; Savitz et al. 1988;
1996; Stevens and Davis 1996; Tenforde 1996; NAS
Coleman et al. 1989; London et al. 1991; Feychting and
1996). Similar reviews have been published on the risk of
Ahlbom 1993; Olsen et al. 1993; Verkasalo et al. 1993;
adverse reproductive outcomes associated with exposure
Michaelis et al. 1997; Linet et al. 1997; Tynes and
to EMF (Chernoff et al. 1992; Brent et al. 1993; Shaw
Haldorsen 1997), all but five reported relative risk
and Croen 1993; NAS 1996; Tenforde 1996).
estimates of between 1.5 and 3.0.
Both direct magnetic field measurements and esti-
Reproductive outcome. Epidemiological studies on
mates based on neighboring power lines are crude proxy
pregnancy outcomes have provided no consistent evi-
measures for the exposure that took place at various
dence of adverse reproductive effects in women working
times before cases of leukemia were diagnosed, and it is
with visual display units (VDUs) (Bergqvist 1993; Shaw
not clear which of the two methods provides the more
and Croen 1993; NRPB 1994a; Tenforde 1996). For
valid estimate. Although results suggest that indeed the
example, meta-analysis revealed no excess risk of spon-
magnetic field may play a role in the association with
taneous abortion or malformation in combined studies
leukemia risk, there is uncertainty because of small
comparing pregnant women using VDUs with women
sample numbers and because of a correlation between the
not using VDUs (Shaw and Croen 1993). Two other
magnetic field and proximity to power lines (Feychting
studies concentrated on actual measurements of the
et al. 1996).
electric and magnetic fields emitted by VDUs; one
Little is known about the etiology of most types of
reported a suggestion of an association between ELF
childhood cancer, but several attempts to control for
magnetic fields and miscarriage (Lindbohm et al. 1992),
potential confounders such as socioeconomic status and
while the other found no such association (Schnorr et al.
air pollution from motor vehicle exhaust fumes have had
1991). A prospective study that included large numbers
little effect on results. Studies that have examined the use
of cases, had high participation rates, and detailed expo-
of electrical appliances (primarily electric blankets) in
sure assessment (Bracken et al. 1995) reported that
relation to cancer and other health problems have re-
neither birth weight nor intra-uterine growth rate was
ported generally negative results (Preston-Martin et al.
related to any ELF field exposure. Adverse outcomes
1988; Verreault et al. 1990; Vena et al. 1991, 1994; Li et
were not associated with higher levels of exposure.
al. 1995). Only two case-control studies have evaluated
Exposure measurements included current-carrying ca-
use of appliances in relation to the risk of childhood
pacity of power lines outside homes, 7-d personal expo-
leukemia. One was conducted in Denver (Savitz et al.
sure measurements, 24-h measurements in the home, and
1990) and suggested a link with prenatal use of electric
self-reported use of electric blankets, heated water beds,
blankets; the other, carried out in Los Angeles (London
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
499
● ICNIRP GUIDELINES
et al. 1991), found an association between leukemia and
which both had not changed their residence during the
children using hair dryers and watching monochrome
years prior to diagnosis. The number of such pairs for
television.
which assessment could be made was 416. There was no
The fact that results for leukemia based on proxim-
indication of an association between wire-code category
ity of homes to power lines are relatively consistent led
and leukemia. As for magnetic field measurements, the
the U.S. National Academy of Sciences Committee to
results are more intriguing. For the cut off point of 0.2
conclude that children living near power lines appear to
T the unmatched and matched analyses gave relative
be at increased risk of leukemia (NAS 1996). Because of
risks of 1.2 and 1.5, respectively. For a cut off point of
small numbers, confidence intervals in the individual
0.3
T, the unmatched relative risk estimate is 1.7 based
studies are wide; when taken together, however, the
on 45 exposed cases. Thus, the measurement results are
results are consistent, with a pooled relative risk of 1.5
suggestive of a positive association between magnetic
(NAS 1996). In contrast, short-term measurements of
fields and leukemia risk. This study is a major contribu-
magnetic field in some of the studies provided no
tion in terms of its size, the number of subjects in high
evidence of an association between exposure to 50/60 Hz
exposure categories, timing of measurements relative to
fields and the risk of leukemia or any other form of
the occurrence of the leukemia (usually within 24 mo
cancer in children. The Committee was not convinced
after diagnosis), other measures used to obtain exposure
that this increase in risk was explained by exposure to
data, and quality of analysis allowing for multiple poten-
magnetic fields, since there was no apparent association
tial confounders. Potential weaknesses include the pro-
when exposure was estimated from magnetic field meter
cedure for control selection, the participation rates, and
readings in the homes of both leukemia cases and
the methods used for statistical analysis of the data. The
controls. It was suggested that confounding by some
instruments used for measurements took no account of
unknown risk factor for childhood leukemia, associated
transient fields or higher order harmonics. The size of this
with residence in the vicinity of power lines, might be the
study is such that its results, combined with those of other
explanation, but no likely candidates were postulated.
studies, would significantly weaken (though not necessarily
After the NAS committee completed its review, the
invalidate) the previously observed association with wire
results of a study performed in Norway were reported
code results.
(Tynes and Haldorsen 1997). This study included 500
Over the years there also has been substantial
interest in whether there is an association between
cases of all types of childhood cancer. Each individual’s
magnetic field exposure and childhood brain cancer, the
exposure was estimated by calculation of the magnetic
second most frequent type of cancer found in children.
field level produced in the residence by nearby transmis-
Three recent studies completed after the NAS Commit-
sion lines, estimated by averaging over an entire year. No
tee’s review fail to provide support for an association
association between leukemia risk and magnetic fields
between brain cancer and children’s exposure to mag-
for the residence at time of diagnosis was observed.
netic fields, whether the source was power lines or
Distance from the power line, exposure during the first
electric blankets, or whether magnetic fields were esti-
year of life, mothers’ exposure at time of conception, and
mated by calculations or by wire codes (Gue´nel et al.
exposure higher than the median level of the controls
1996; Preston-Martin et al. 1996a, b; Tynes and Hal-
showed no association with leukemia, brain cancer, or
dorsen 1997).
lymphoma. However, the number of exposed cases was
Data on cancer in adults and residential magnetic
small.
field exposure are sparse (NAS 1996). The few studies
Also, a study performed in Germany has been
published to date (Wertheimer and Leeper 1979; Mc-
reported after the completion of the NAS review
Dowall 1985; Seversen et al. 1988; Coleman et al. 1989;
(Michaelis et al. 1997). This was a case-control study on
Schreiber et al. 1993; Feychting and Ahlbom 1994; Li et
childhood leukemia based on 129 cases and 328 controls.
al. 1996; Verkasalo 1996; Verkasalo et al. 1996) all
Exposure assessment comprised measurements of the
suffer to some extent from small numbers of exposed
magnetic field over 24 h in the child’s bedroom at the
cases, and no conclusions can be drawn.
residence where the child had been living for the longest
It is the view of the ICNIRP that the results from the
period before the date of diagnosis. An elevated relative
epidemiological research on EMF field exposure and
risk of 3.2 was observed for
0.2
T.
cancer, including childhood leukemia, are not strong
A large U.S. case-control study (638 cases and 620
enough in the absence of support from experimental re-
controls) to test whether childhood acute lymphoblastic
search to form a scientific basis for setting exposure
leukemia is associated with exposure to 60-Hz magnetic
guidelines. This assessment is also in agreement with recent
fields was published by Linet et al. (1997). Magnetic
reviews (NRPB 1992, 1994b; NAS 1996; CRP 1997).
field exposures were determined using 24-h time-
weighted average measurements in the bedroom and 30-s
Occupational studies. A large number of epidemi-
measurements in various other rooms. Measurements
ological studies have been carried out to assess possible
were taken in homes in which the child had lived for 70%
links between exposure to ELF fields and cancer risk
of the 5 y prior to the year of diagnosis, or the
among workers in electrical occupations. The first study
corresponding period for the controls. Wire-codes were
of this type (Milham 1982) took advantage of a death
assessed for residentially stable case-control pairs in
certificate database that included both job titles and
500
Health Physics
April 1998, Volume 74, Number 4
information on cancer mortality. As a crude method of
An association between Alzheimer’s disease and
assessing exposure, Milham classified job titles accord-
occupational exposure to magnetic fields has recently
ing to presumed magnetic field exposure and found an
been suggested (Sobel and Davanipour 1996). However,
excess risk for leukemia among electrical workers. Sub-
this effect has not been confirmed.
sequent studies (Savitz and Ahlbom 1994) made use of
similar databases; the types of cancer for which elevated
Laboratory studies. The following paragraphs pro-
rates were noted varied across studies, particularly when
vide a summary and critical evaluation of laboratory
cancer subtypes were characterized. Increased risks of
studies on the biological effects of electric and magnetic
various types of leukemia and nervous tissue tumors,
fields with frequencies below 100 kHz. There are sepa-
and, in a few instances, of both male and female breast
rate discussions on results obtained in studies of volun-
cancer, were reported (Demers et al. 1991; Matanoski et
teers exposed under controlled conditions and in labora-
al. 1991; Tynes et al. 1992; Loomis et al. 1994). As well
tory studies on cellular, tissue, and animal systems.
as producing somewhat inconsistent results, these studies
suffered from very crude exposure assessment and from
failure to control for confounding factors such as expo-
Volunteer studies. Exposure to a time-varying elec-
sure to benzene solvent in the workplace.
tric field can result in perception of the field as a result of
Three recent studies have attempted to overcome
the alternating electric charge induced on the body
some of the deficiencies in earlier work by measuring
surface, which causes the body hairs to vibrate. Several
ELF field exposure at the workplace and by taking
studies have shown that the majority of people can
duration of work into consideration (Floderus et al. 1993;
perceive
50/60
Hz
electric
fields
stronger
than
The´riault et al. 1994; Savitz and Loomis 1995). An
20 kV m 1, and that a small minority can perceive fields
elevated cancer risk among exposed individuals was
below 5 kV m 1 (UNEP/WHO/IRPA 1984; Tenforde
observed, but the type of cancer of which this was true
1991).
varied from study to study. Floderus et al. (1993) found
Small changes in cardiac function occurred in hu-
man volunteers exposed to combined 60-Hz electric and
a significant association with leukemia; an association
magnetic fields (9 kV m 1, 20
T) (Cook et al. 1992;
was also noted by The´riault et al. (1994), but one that was
Graham et al. 1994). Resting heart rate was slightly, but
weak and not significant, and no link was observed by
significantly, reduced (by 3–5 beats per minute) during
Savitz and Loomis (1995). For subtypes of leukemia
or immediately after exposure. This response was absent
there was even greater inconsistency, but numbers in the
on exposure to stronger (12 kV m 1, 30
T) or weaker
analyses were small. For tumors of nervous tissue,
(6 kV m 1, 10
T) fields and reduced if the subject was
Floderus et al. (1993) found an excess for glioblastoma
mentally alert. None of the subjects in these studies was
(astrocytoma III–IV), while both The´riault et al. (1994)
able to detect the presence of the fields, and there were
and Savitz and Loomis (1995) found only suggestive
no other consistent results in a wide battery of sensory
evidence for an increase in glioma (astrocytoma I–II). If
and perceptual tests.
there is truly a link between occupational exposure to
No adverse physiological or psychological effects
magnetic fields and cancer, greater consistency and
were observed in laboratory studies of people exposed to
stronger associations would be expected of these recent
50-Hz fields in the range 2–5 mT (Sander et al. 1982;
studies based on more sophisticated exposure data.
Ruppe et al. 1995). There were no observed changes in
Researchers have also investigated the possibility
blood chemistry, blood cell counts, blood gases, lactate
that ELF electric fields could be linked to cancer. The
levels, electrocardiogram, electroencephalogram, skin
three utilities that participated in the The´riault et al.
temperature, or circulating hormone levels in studies by
(1994) study of magnetic fields analyzed electric field
Sander et al. (1982) and Graham et al. (1994). Recent
data as well. Workers with leukemia at one of the utilities
studies on volunteers have also failed to show any effect
were reported to be more likely to have been exposed to
of exposure to 60-Hz magnetic fields on the nocturnal
electric fields than were control workers. In addition, the
melatonin level in blood (Graham et al. 1996, 1997;
association was stronger in a group that had been
Selmaoui et al. 1996).
exposed to high electric and magnetic fields combined
Sufficiently intense ELF magnetic fields can elicit
(Miller et al. 1996). At the second utility, investigators
peripheral nerve and muscle tissue stimulation directly,
reported no association between leukemia and higher
and short magnetic field pulses have been used clinically
cumulative exposure to workplace electric fields, but
to stimulate nerves in the limbs in order to check the
some of the analyses showed an association with brain
integrity of neural pathways. Peripheral nerve and mus-
cancer (Gue´nel et al. 1996). An association with colon
cle stimulation has also been reported in volunteers
cancer was also reported, yet in other studies of large
exposed to 1-kHz gradient magnetic fields in experimen-
populations of electric utility workers this type of cancer
tal magnetic resonance imaging systems. Threshold mag-
has not been found. At the third utility, no association
netic flux densities were several millitesla, and corre-
between high electric fields and brain cancer or leukemia
sponding induced current densities in the peripheral
was observed, but this study was smaller and less likely
tissues were about 1 A m 2 from pulsed fields produced
to have detected small changes, if present (Baris et al.
by rapidly switched gradients. Time-varying magnetic
1996).
fields that induce current densities above 1 A m 2 in
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
501
● ICNIRP GUIDELINES
tissue lead to neural excitation and are capable of
ple models of the behavior of single cells in weak fields
producing irreversible biological effects such as cardiac
it has been calculated that an electrical signal in the
fibrillation (Tenforde and Kaune 1987; Reilly 1989). In a
extracellular field must be greater than approximately
study involving electromyographic recordings from the
10 –100 mV m 1 (corresponding to an induced current
human arm (Polson et al. 1982), it was found that a
density of about 2–20 mA m 2) in order to exceed the
pulsed field with dB/dt greater than 104 T s 1 was
level of endogenous physical and biological noise in
needed to stimulate the median nerve trunk. The duration
cellular membranes (Astumian et al. 1995). Existing
of the magnetic stimulus has also been found to be an
evidence also suggests that several structural and func-
important parameter in stimulation of excitable tissues.
tional properties of membranes may be altered in re-
Thresholds lower than 100 mA m 2 can be derived
sponse to induced ELF fields at or below 100 mV m 1
from studies of visual and mental functions in human
(Sienkiewicz et al. 1991; Tenforde 1993). Neuroendo-
volunteers. Changes in response latency for complex
crine alterations (e.g., suppression of nocturnal melatonin
reasoning tests have been reported in volunteers
synthesis) have been reported in response to induced
subjected to weak power-frequency electric currents
electrical fields of 10 mV m 1 or less, corresponding to
passed through electrodes attached to the head and
induced current densities of approximately 2 mA m 2 or
shoulders; current densities were estimated to lie be-
less (Tenforde 1991, 1996). However, there is no clear
tween 10 and 40 mA m 2 (Stollery 1986, 1987). Finally,
evidence that these biological interactions of low-
many studies have reported that volunteers experienced
frequency fields lead to adverse health effects.
faint flickering visual sensations, known as magnetic
Induced electric fields and currents at levels exceed-
phosphenes, during exposure to ELF magnetic fields
ing those of endogenous bioelectric signals present in
above 3–5 mT (Silny 1986). These visual effects can also
tissue have been shown to cause a number of physiolog-
be induced by the direct application of weak electric
ical effects that increase in severity as the induced current
currents to the head. At 20 Hz, current densities of about
density is increased (Bernhardt 1979; Tenforde 1996). In
10 mA m 2 in the retina have been estimated as the
the current density range 10 –100 mA m 2, tissue effects
threshold for induction of phosphenes, which is above
and changes in brain cognitive functions have been
the typical endogenous current densities in electrically
reported (NRPB 1992; NAS 1996). When induced cur-
excitable tissues. Higher thresholds have been observed
rent density exceeds 100 to several hundred mA m 2 for
for both lower and higher frequencies (Lo¨vsund et al.
frequencies between about 10 Hz and 1 kHz, thresholds
1980; Tenforde 1990).
for neuronal and neuromuscular stimulation are ex-
Studies have been conducted at 50 Hz on visually
ceeded. The threshold current densities increase progres-
evoked potentials that exhibited thresholds for effects at
sively at frequencies below several hertz and above
flux densities of 60 mT (Silny 1986). Consistent with this
1 kHz. Finally, at extremely high current densities,
result, no effects on visually evoked potentials were
exceeding 1 A m 2, severe and potentially life-
obtained by either Sander et al. (1982), using a 50-Hz,
threatening effects such as cardiac extrasystoles, ventric-
5-mT field, or Graham et al. (1994), using combined
ular fibrillation, muscular tetanus, and respiratory failure
60-Hz electric and magnetic fields up to 12 kV m 1 and
30
T, respectively.
may occur. The severity and the probability of irrevers-
ibility of tissue effects becomes greater with chronic
exposure to induced current densities above the level
Cellular and animal studies. Despite the large
10 to 100 mA m 2. It therefore seems appropriate to
number of studies undertaken to detect biological effects
limit human exposure to fields that induce current den-
of ELF electric and magnetic fields, few systematic
sities no greater than 10 mA m 2 in the head, neck, and
studies have defined the threshold field characteristics
trunk at frequencies of a few hertz up to 1 kHz.
that produce significant perturbations of biological func-
It has been postulated that oscillatory magnetome-
tions. It is well established that induced electric current
chanical forces and torques on biogenic magnetite par-
can stimulate nerve and muscle tissue directly once the
ticles in brain tissue could provide a mechanism for
induced
current
density
exceeds
threshold
values
the transduction of signals from ELF magnetic fields.
(UNEP/WHO/IRPA 1987; Bernhardt 1992; Tenforde
Kirschvink et al. (1992b) proposed a model in which
1996). Current densities that are unable to stimulate
ELF magnetic forces on magnetite particles are visual-
excitable tissues directly may nevertheless affect ongo-
ized as producing the opening and closing of pressure-
ing electrical activity and influence neuronal excitability.
sensitive ion channels in membranes. However, one
The activity of the central nervous system is known to be
difficulty with this model is the sparsity of magnetite
sensitive to the endogenous electric fields generated by
particles relative to the number of cells in brain tissue.
the action of adjacent nerve cells, at levels below those
For example, human brain tissue has been reported to
required for direct stimulation.
contain a few million magnetite particles per gram,
Many studies have suggested that the transduction
distributed in 105 discrete clusters of 5–10 particles
of weak electrical signals in the ELF range involves
(Kirschvink et al. 1992a). The number of cells in brain
interactions with the cell membrane, leading to cytoplas-
tissue thus exceeds the number of magnetite particles by
mic biochemical responses that in turn involve changes
a factor of about 100, and it is difficult to envisage how
in cellular functional and proliferative states. From sim-
oscillating magnetomechanical interactions of an ELF
502
Health Physics
April 1998, Volume 74, Number 4
field with magnetite crystals could affect a significant
tional and neoplastic transformation effects are expected.
number of pressure-sensitive ion channels in the brain.
This is supported by results of laboratory studies de-
Further studies are clearly needed to reveal the biological
signed to detect DNA and chromosomal damage, muta-
role of magnetite and the possible mechanisms through
tional events, and increased transformation frequency in
which this mineral could play a role in the transduction of
response to ELF field exposure (NRPB 1992; Murphy et
ELF magnetic signals.
al. 1993; McCann et al. 1993; Tenforde 1996). The lack
An important issue in assessing the effects of elec-
of effects on chromosome structure suggests that ELF
tromagnetic fields is the possibility of teratogenic and
fields, if they have any effect on the process of carcino-
developmental effects. On the basis of published scien-
genesis, are more likely to act as promoters than initia-
tific evidence, it is unlikely that low-frequency fields
tors, enhancing the proliferation of genetically altered
have adverse effects on the embryonic and postnatal
cells rather than causing the initial lesion in DNA or
development of mammalian species (Chernoff et al.
chromatin. An influence on tumor development could be
1992; Brent et al. 1993; Tenforde 1996). Moreover,
mediated through epigenetic effects of these fields, such
currently available evidence indicates that somatic mu-
as alterations in cell signalling pathways or gene expres-
tations and genetic effects are unlikely to result from
sion. The focus of recent studies has therefore been on
exposure to electric and magnetic fields with frequencies
detecting possible effects of ELF fields on the promotion
below 100 kHz (Cridland 1993; Sienkiewicz et al. 1993).
and progression phases of tumor development following
There are numerous reports in the literature on the
initiation by a chemical carcinogen.
in-vitro effects of ELF fields on cell membrane proper-
Studies on in-vitro tumor cell growth and the devel-
ties (ion transport and interaction of mitogens with cell
opment of transplanted tumors in rodents have provided
surface receptors) and changes in cellular functions and
no strong evidence for possible carcinogenic effects of
growth properties (e.g., increased proliferation and alter-
exposure to ELF fields (Tenforde 1996). Several studies
ations in metabolism, gene expression, protein biosyn-
of more direct relevance to human cancer have involved
thesis, and enzyme activities) (Cridland 1993; Sien-
in-vivo tests for tumor-promoting activity of ELF mag-
kiewicz et al. 1993; Tenforde 1991, 1992, 1993, 1996).
netic fields on skin, liver, brain, and mammary tumors in
Considerable attention has focused on low-frequency
rodents. Three studies of skin tumor promotion (McLean
field effects on Ca
transport across cell membranes
et al. 1991; Rannug et al. 1993a, 1994) failed to show any
and the intracellular concentration of this ion (Walleczek
effect of either continuous or intermittent exposure to
and Liburdy 1990; Liburdy 1992; Walleczek 1992),
power-frequency magnetic fields in promoting chemi-
messenger RNA and protein synthesis patterns (Good-
cally induced tumors. At a 60-Hz field strength of 2 mT,
man et al. 1983; Goodman and Henderson 1988, 1991;
a co-promoting effect with a phorbol ester was reported
Greene et al. 1991; Phillips et al. 1992), and the activity
for mouse skin tumor development in the initial stages of
of enzymes such as ornithine decarboxylase (ODC) that
the experiment, but the statistical significance of this was
are related to cell proliferation and tumor promotion
lost by completion of the study in week 23 (Stuchly et al.
(Byus et al. 1987, 1988; Litovitz et al. 1991, 1993).
1992). Previous studies by the same investigators had
However, before these observations can be used for
shown that 60-Hz, 2-mT field exposure did not promote
defining exposure limits, it is essential to establish both
the growth of DMBA-initiated skin cells (McLean et al.
their reproducibility and their relevance to cancer or
1991).
other adverse health outcomes. This point is underscored
Experiments on the development of transformed
by the fact that there have been difficulties in replicating
liver foci initiated by a chemical carcinogen and pro-
some of the key observations of field effects on gene
moted by phorbol ester in partially hepatectomized rats
expression and protein synthesis (Lacy-Hulbert et al.
revealed no promotion or co-promotion effect of expo-
1995; Saffer and Thurston 1995). The authors of these
sure to 50-Hz fields ranging in strength from 0.5 to 50
replication studies identified several deficiencies in the
T (Rannug et al. 1993b, c).
earlier studies, including poor temperature control, lack
Studies on mammary cancer development in rodents
of appropriate internal control samples, and the use of
treated with a chemical initiator have suggested a cancer-
low-resolution techniques for analyzing the production
promoting effect of exposure to power-frequency mag-
of messenger RNA transcripts. The transient increase in
netic fields in the range 0.01–30 mT (Beniashvili et al.
ODC activity reported in response to field exposure is
1991; Lo¨scher et al. 1993; Mevissen et al. 1993, 1995;
small in magnitude and not associated with de novo
Baum et al. 1995; Lo¨scher and Mevissen 1995). These
synthesis of the enzyme (unlike chemical tumor promot-
observations of increased tumor incidence in rats ex-
ers such as phorbol esters) (Byus et al. 1988). Studies on
posed to magnetic fields have been hypothesized to be
ODC have mostly involved cellular preparations; more
related to field-induced suppression of pineal melatonin
studies are needed to show whether there are effects on
and a resulting elevation in steroid hormone levels and
ODC in vivo, although there is one report suggesting
breast cancer risk (Stevens 1987; Stevens et al. 1992).
effects on ODC in a rat mammary tumor promotion assay
However, replication efforts by independent laboratories
(Mevissen et al. 1995).
are needed before conclusions can be drawn regarding
There is no evidence that ELF fields alter the
the implications of these findings for a promoting effect
structure of DNA and chromatin, and no resultant muta-
of ELF magnetic fields on mammary tumors. It should
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
503
● ICNIRP GUIDELINES
also be noted that recent studies have found no evidence
Summary of biological effects and epidemiological
for a significant effect of exposure to ELF magnetic
studies (up to 100 kHz)
fields on melatonin levels in humans (Graham et al.
With the possible exception of mammary tumors,
1996, 1997; Selmaoui et al. 1996).
there is little evidence from laboratory studies that
power-frequency
magnetic
fields
have
a
tumor-
promoting effect. Although further animal studies are
Indirect effects of electric and magnetic fields
needed to clarify the possible effects of ELF fields on
Indirect effects of electromagnetic fields may result
signals produced in cells and on endocrine regulation—
from physical contact (e.g., touching or brushing against)
both of which could influence the development of tumors
between a person and an object, such as a metallic
by promoting the proliferation of initiated cells—it can
structure in the field, at a different electric potential. The
result of such contact is the flow of electric charge
only be concluded that there is currently no convincing
(contact current) that may have accumulated on the
evidence for carcinogenic effects of these fields and that
object or on the body of the person. In the frequency
these data cannot be used as a basis for developing
range up to approximately 100 kHz, the flow of electric
exposure guidelines.
current from an object in the field to the body of the
Laboratory studies on cellular and animal systems
individual may result in the stimulation of muscles
have found no established effects of low-frequency fields
and/or peripheral nerves. With increasing levels of cur-
that are indicative of adverse health effects when induced
rent this may be manifested as perception, pain from
current density is at or below 10 mA m 2. At higher
electric shock and/or burn, inability to release the object,
levels of induced current density (10 –100 mA m 2),
difficulty in breathing and, at very high currents, cardiac
more significant tissue effects have been consistently
ventricular fibrillation (Tenforde and Kaune 1987).
observed, such as functional changes in the nervous
Threshold values for these effects are frequency-
system and other tissue effects (Tenforde 1996).
dependent, with the lowest threshold occurring at fre-
Data on cancer risk associated with exposure to ELF
quencies between 10 and 100 Hz. Thresholds for periph-
fields among individuals living close to power lines are
eral nerve responses remain low for frequencies up to
apparently consistent in indicating a slightly higher risk
several kHz. Appropriate engineering and/or administra-
of leukemia among children, although more recent stud-
tive controls, and even the wearing of personal protective
ies question the previously observed weak association.
clothing, can prevent these problems from occurring.
The studies do not, however, indicate a similarly elevated
Spark discharges can occur when an individual
risk of any other type of childhood cancer or of any form
comes into very close proximity with an object at a
of adult cancer. The basis for the hypothetical link
different electric potential, without actually touching it
between childhood leukemia and residence in close
(Tenforde and Kaune 1987; UNEP/WHO/IRPA 1993).
proximity to power lines is unknown; if the link is not
When a group of volunteers, who were electrically
related to the ELF electric and magnetic fields generated
insulated from the ground, each held a finger tip close to
by the power lines, then unknown risk factors for
a grounded object, the threshold for perception of spark
leukemia would have to be linked to power lines in some
discharges was as low as 0.6 –1.5 kV m 1 in 10% of
undetermined manner. In the absence of support from
cases. The threshold field level reported as causing
laboratory studies, the epidemiological data are insuffi-
annoyance under these exposure conditions is about
cient to allow an exposure guideline to be established.
2.0 –3.5 kV m 1. Large contact currents can result in
There have been reports of an increased risk of
muscle contraction. In male volunteers, the 50th percen-
certain types of cancer, such as leukemia, nervous tissue
tile threshold for being unable to release a charged
tumors, and, to a limited extent, breast cancer, among
conductor has been reported as 9 mA at 50/60 Hz, 16 mA
electrical workers. In most studies, job titles were used to
at 1 kHz, about 50 mA at 10 kHz, and about 130 mA at
classify subjects according to presumed levels of mag-
100 kHz (UNEP/WHO/IRPA 1993).
netic field exposure. A few more recent studies, however,
The threshold currents for various indirect effects of
have used more sophisticated methods of exposure assess-
fields with frequencies up to 100 kHz are summarized in
ment; overall, these studies suggested an increased risk of
Table 2 (UNEP/WHO/IRPA 1993).
leukemia or brain tumors but were largely inconsistent with
regard to the type of cancer for which risk is increased. The
data are insufficient to provide a basis for ELF field
exposure guidelines. In a large number of epidemiological
Table 2. Ranges of threshold currents for indirect effects, includ-
studies, no consistent evidence of adverse reproductive
ing children, women, and men.
effects have been provided.
Threshold current (mA) at
Measurement of biological responses in laboratory
frequency:
studies and in volunteers has provided little indication of
Indirect effect
50/60 Hz
1 kHz
100 kHz
adverse effects of low-frequency fields at levels to which
Touch perception
0.2–0.4
0.4–0.8
25–40
people are commonly exposed. A threshold current
Pain on finger contact
0.9–1.8
1.6–3.3
33–55
density of 10 mA m 2 at frequencies up to 1 kHz has
Painful shock/let-go threshold
8–16
12–24
112–224
been estimated for minor effects on nervous system
Severe shock/breathing difficulty
12–23
21–41
160–320
functions. Among volunteers, the most consistent effects
504
Health Physics
April 1998, Volume 74, Number 4
of exposure are the appearance of visual phosphenes and
Cancer studies. Studies on cancer risk and micro-
a minor reduction in heart rate during or immediately
wave exposure are few and generally lack quantitative
after exposure to ELF fields, but there is no evidence that
exposure assessment. Two epidemiological studies of
these transient effects are associated with any long-term
radar workers in the aircraft industry and in the U.S.
health risk. A reduction in nocturnal pineal melatonin
armed forces found no evidence of increased morbidity
synthesis has been observed in several rodent species
or mortality from any cause (Barron and Baraff 1958;
following exposure to weak ELF electric and magnetic
Robinette et al. 1980; UNEP/WHO/IRPA 1993). Similar
fields, but no consistent effect has been reported in
results were obtained by Lillienfeld et al. (1978) in a
humans exposed to ELF fields under controlled condi-
study of employees in the U.S. embassy in Moscow, who
tions. Studies involving exposures to 60-Hz magnetic
were chronically exposed to low-level microwave radia-
fields up to 20
T have not reported reliable effects on
tion. Selvin et al. (1992) reported no increase in cancer
melatonin levels in blood.
risk among children chronically exposed to radiation
from a large microwave transmitter near their homes.
More recent studies have failed to show significant
BIOLOGICAL BASIS FOR LIMITING
increases in nervous tissue tumors among workers and
EXPOSURE (100 k H z –300 GHz)
military personnel exposed to microwave fields (Beall et
The following paragraphs provide a general review
al. 1996; Grayson 1996). Moreover, no excess total
of relevant literature on the biological effects and poten-
mortality was apparent among users of mobile tele-
tial health effects of electromagnetic fields with frequen-
phones (Rothman et al. 1996a, b), but it is still too early
cies of 100 kHz to 300 GHz. More detailed reviews can
to observe an effect on cancer incidence or mortality.
be found elsewhere (NRPB 1991; UNEP/WHO/IRPA
There has been a report of increased cancer risk
1993; McKinlay et al. 1996; Polk and Postow 1996;
among military personnel (Szmigielski et al. 1988), but
Repacholi 1998).
the results of the study are difficult to interpret because
neither the size of the population nor the exposure levels
are clearly stated. In a later study, Szmigielski (1996)
Direct effects of electromagnetic fields
found increased rates of leukemia and lymphoma among
military personnel exposed to EMF fields, but the assess-
Epidemiological studies. Only a limited number of
ment of EMF exposure was not well defined. A few
studies have been carried out on reproductive effects and
recent studies of populations living near EMF transmit-
cancer risk in individuals exposed to microwave radia-
ters have suggested a local increase in leukemia inci-
tion. A summary of the literature was published by
dence (Hocking et al. 1996; Dolk et at. 1997a, b), but the
UNEP/WHO/IRPA (1993).
results are inconclusive. Overall, the results of the small
number of epidemiological studies published provide
Reproductive outcomes. Two extensive studies on
only limited information on cancer risk.
women treated with microwave diathermy to relieve the
pain of uterine contractions during labor found no evi-
Laboratory studies. The following paragraphs pro-
dence for adverse effects on the fetus (Daels 1973, 1976).
vide a summary and critical evaluation of laboratory
However, seven studies on pregnancy outcomes among
studies on the biological effects of electromagnetic fields
workers occupationally exposed to microwave radiation
with frequencies in the range 100 kHz–300 GHz. There
and on birth defects among their offspring produced both
are separate discussions on results of studies of volun-
positive and negative results. In some of the larger
teers exposed under controlled conditions and of labora-
epidemiological studies of female plastic welders and
tory studies on cellular, tissue, and animal systems.
physiotherapists working with shortwave diathermy de-
vices, there were no statistically significant effects on
rates of abortion or fetal malformation (Ka¨llen et al.
Volunteer studies. Studies by Chatterjee et al.
1982). By contrast, other studies on similar populations
(1986) demonstrated that, as the frequency increases
of female workers found an increased risk of miscarriage
from approximately 100 kHz to 10 MHz, the dominant
and birth defects (Larsen et al. 1991; Ouellet-Hellstrom
effect of exposure to a high-intensity electromagnetic
and Stewart 1993). A study of male radar workers found
field changes from nerve and muscle stimulation to
no association between microwave exposure and the risk
heating. At 100 kHz the primary sensation was one of
of Down’s syndrome in their offspring (Cohen et al.
nerve tingling, while at 10 MHz it was one of warmth on
1977).
the skin. In this frequency range, therefore, basic health
Overall, the studies on reproductive outcomes and
protection criteria should be such as to avoid stimulation
microwave exposure suffer from very poor assessment of
of excitable tissues and heating effects. At frequencies
exposure and, in many cases, small numbers of subjects.
from 10 MHz to 300 GHz, heating is the major effect of
Despite the generally negative results of these studies, it
absorption of electromagnetic energy, and temperature
will be difficult to draw firm conclusions on reproductive
rises of more than 1–2 °C can have adverse health effects
risk without further epidemiological data on highly
such as heat exhaustion and heat stroke (ACGIH 1996).
exposed individuals and more precise exposure assess-
Studies on workers in thermally stressful environments
ment.
have shown worsening performance of simple tasks as
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
505
● ICNIRP GUIDELINES
body temperature rises to a level approaching physiolog-
monkeys, altered thermoregulatory behavior starts when
ical heat stress (Ramsey and Kwon 1988).
the temperature in the hypothalamic region rises by as
A sensation of warmth has been reported by volun-
little as 0.2– 0.3°C (Adair et al. 1984). The hypothalamus
teers experiencing high-frequency current of about 100 –
is considered to be the control center for normal thermo-
200 mA through a limb. The resulting SAR value is
regulatory processes, and its activity can be modified by
unlikely to produce a localized temperature increment of
a small local temperature increase under conditions in
more than 1°C in the limbs (Chatterjee et al. 1986; Chen
which rectal temperature remains constant.
and Gandhi 1988; Hoque and Gandhi 1988), which has
At levels of absorbed electromagnetic energy that
been suggested as the upper limit of temperature increase
cause body temperature rises in excess of 1–2°C, a large
that has no detrimental health effects (UNEP/WHO/
number of physiological effects have been characterized
IRPA 1993). Data on volunteers reported by Gandhi et al.
in studies with cellular and animal systems (Michaelson
(1986) for frequencies up to 50 MHz and by Tofani et al.
and Elson 1996). These effects include alterations in
(1995) for frequencies up to 110 MHz (the upper limit of
neural and neuromuscular functions; increased blood-
the FM broadcast band) support a reference level for limb
brain barrier permeability; ocular impairment (lens opac-
current of 100 mA to avoid excessive heating effects
ities
and
corneal
abnormalities);
stress-associated
(Dimbylow 1997).
changes in the immune system; hematological changes;
There have been several studies of thermoregulatory
reproductive changes (e.g., reduced sperm production);
responses of resting volunteers exposed to EMF in
teratogenicity; and changes in cell morphology, water
magnetic resonance imaging systems (Shellock and
and electrolyte content, and membrane functions.
Crues 1987; Magin et al. 1992). In general, these have
Under conditions of partial-body exposure to intense
demonstrated that exposure for up to 30 min, under
EMF, significant thermal damage can occur in sensitive
conditions in which whole-body SAR was less than
tissues such as the eye and the testis. Microwave expo-
4 W kg 1, caused an increase in the body core temper-
sure of 2–3 h duration has produced cataracts in rabbits’
ature of less than 1°C.
eyes at SAR values from 100 –140 W kg 1, which
produced lenticular temperatures of 41– 43°C (Guy et al.
1975). No cataracts were observed in monkeys exposed
Cellular and animal studies. There are numerous
to microwave fields of similar or higher intensities,
reports on the behavioral and physiological responses of
possibly because of different energy absorption patterns
laboratory animals, including rodents, dogs, and non-
in the eyes of monkeys from those in rabbits. At very
human primates, to thermal interactions of EMF at
high frequencies (10 –300 GHz), absorption of electro-
frequencies above 10 MHz. Thermosensitivity and ther-
magnetic energy is confined largely to the epidermal
moregulatory responses are associated both with the
layers of the skin, subcutaneous tissues, and the outer
hypothalamus and with thermal receptors located in the
skin and in internal parts of the body. Afferent signals
part of the eye. At the higher end of the frequency range,
reflecting temperature change converge in the central
absorption is increasingly superficial. Ocular damage at
nervous system and modify the activity of the major
these frequencies can be avoided if the microwave power
neuroendocrine control systems, triggering the physio-
density is less than 50 W m 2 (Sliney and Wolbarsht
logical and behavioral responses necessary for the main-
1980; UNEP/WHO/IRPA 1993).
tenance of homeostasis.
There has been considerable recent interest in the
Exposure of laboratory animals to EMF producing
possible carcinogenic effects of exposure to microwave
absorption in excess of approximately 4 W kg 1 has
fields with frequencies in the range of widely used
revealed a characteristic pattern of thermoregulatory
communications systems, including hand-held mobile
response in which body temperature initially rises and
telephones and base transmitters. Research findings in
then stabilizes following the activation of thermoregula-
this area have been summarized by ICNIRP (1996).
tory mechanisms (Michaelson 1983). The early phase of
Briefly, there are many reports suggesting that micro-
this response is accompanied by an increase in blood
wave fields are not mutagenic, and exposure to these
volume due to movement of fluid from the extracellular
fields is therefore unlikely to initiate carcinogenesis
space into the circulation and by increases in heart rate
(NRPB 1992; Cridland 1993; UNEP/WHO/IRPA 1993).
and intraventricular blood pressure. These cardiody-
By contrast, some recent reports suggest that exposure of
namic changes reflect thermoregulatory responses that
rodents to microwave fields at SAR levels of the order of
facilitate the conduction of heat to the body surface.
1 W kg 1 may produce strand breaks in the DNA of
Prolonged exposure of animals to levels of microwave
testis and brain tissues (Sarkar et al. 1994; Lai and Singh
radiation that raise the body temperature ultimately lead
1995, 1996), although both ICNIRP (1996) and Williams
to failure of these thermoregulatory mechanisms.
(1996) pointed out methodological deficiencies that
Several studies with rodents and monkeys have also
could have significantly influenced these results.
demonstrated a behavioral component of thermoregula-
In a large study of rats exposed to microwaves for
tory responses. Decreased task performance by rats and
up to 25 mo, an excess of primary malignancies was
monkeys has been observed at SAR values in the range
noted in exposed rats relative to controls (Chou et al.
1–3 W kg 1 (Stern et al. 1979; Adair and Adams 1980;
1992). However, the incidence of benign tumors did not
de Lorge and Ezell 1980; D’Andrea et al. 1986). In
differ between the groups, and no specific type of tumor
506
Health Physics
April 1998, Volume 74, Number 4
was more prevalent in the exposed group than in stock
Some reports suggest that retina, iris, and corneal
rats of the same strain maintained under similar specific-
endothelium of the primate eye are sensitive to low levels
pathogen-free conditions. Taken as a whole, the results
of pulsed microwave radiation (Kues et al. 1985; UNEP/
of this study cannot be interpreted as indicating a
WHO/IRPA 1993). Degenerative changes in light-
tumor-initiating effect of microwave fields.
sensitive cells of the retina were reported for absorbed
Several studies have examined the effects of micro-
energy levels as low as 26 mJ kg 1. After administration
wave exposure on the development of pre-initiated tumor
of timolol maleate, which is used in the treatment of
cells. Szmigielski et al. (1982) noted an enhanced growth
glaucoma, the threshold for retinal damage by pulsed
rate of transplanted lung sarcoma cells in rats exposed to
fields dropped to 2.6 mJ kg 1. However, an attempt in an
microwaves at high power densities. It is possible that
independent laboratory to partially replicate these find-
this resulted from a weakening of the host immune
ings for CW fields (i.e., not pulsed) was unsuccessful
defense in response to thermal stress from the microwave
(Kamimura et al. 1994), and it is therefore impossible at
exposure. Recent studies using athermal levels of micro-
present to assess the potential health implications of the
wave irradiation have found no effects on the develop-
initial findings of Kues et al. (1985).
ment of melanoma in mice or of brain glioma in rats
Exposure to intense pulsed microwave fields has
(Santini et al. 1988; Salford et al. 1993).
been reported to suppress the startle response in con-
Repacholi et al. (1997) have reported that exposure
scious mice and to evoke body movements (NRPB 1991;
of 100 female, E -pim1 transgenic mice to 900-MHz
Sienkiewicz et al. 1993; UNEP/WHO/IRPA 1993). The
fields, pulsed at 217 Hz with pulse widths of 0.6
s for
threshold specific energy absorption level at midbrain
up to 18 mo, produced a doubling in lymphoma inci-
that evoked body movements was 200 J kg 1 for 10
s
dence compared with 101 controls. Because the mice
pulses. The mechanism for these effects of pulsed mi-
were free to roam in their cages, the variation in SAR
crowaves remains to be determined but is believed to be
was wide (0.01– 4.2 W kg 1). Given that the resting
related to the microwave hearing phenomenon. The
metabolic rate of these mice is 7–15 W kg 1, only the
auditory thresholds for rodents are about an order of
upper end of the exposure range may have produced
magnitude lower than for humans, that is 1–2 mJ kg 1
some slight heating. Thus, it appears that this study
for pulses
30
s in duration. Pulses of this magnitude
suggests a non-thermal mechanism may be acting, which
have also been reported to affect neurotransmitter me-
needs to be investigated further. However, before any
tabolism and the concentration of the neural receptors
assumptions can be made about health risk, a number of
involved in stress and anxiety responses in different
questions need to be addressed. The study needs to be
regions of the rat brain.
replicated, restraining the animals to decrease the SAR
The issue of athermal interactions of high-frequency
exposure variation and to determine whether there is a
EMF has centered largely on reports of biological effects
dose response. Further study is needed to determine
of amplitude modulated (AM) fields under in-vitro con-
whether the results can be found in other animal models
ditions at SAR values well below those that produce
in order to be able to generalize the results to humans. It
measurable tissue heating. Initial studies in two indepen-
is also essential to assess whether results found in
transgenic animals are applicable to humans.
dent laboratories led to reports that VHF fields with
amplitude modulation at extremely low frequencies
Special considerations for pulsed and
(6 –20 Hz) produced a small, but statistically significant,
amplitude-modulated waveforms
release of Ca
from the surfaces of chick brain cells
Compared with continuous-wave (CW) radiation,
(Bawin et al. 1975; Blackman et al. 1979). A subsequent
pulsed microwave fields with the same average rate of
attempt to replicate these findings, using the same type of
energy deposition in tissues are generally more effective
AM field, was unsuccessful (Albert et al. 1987). A
in producing a biological response, especially when there
number of other studies of the effects of AM fields on
is a well-defined threshold that must be exceeded to elicit
Ca
homeostasis have produced both positive and
the effect (ICNIRP 1996). The “microwave hearing”
negative results. For example, effects of AM fields on
effect is a well known example of this (Frey 1961; Frey
Ca
binding to cell surfaces have been observed with
and Messenger 1973; Lin 1978): people with normal
neuroblastoma cells, pancreatic cells, cardiac tissue, and
hearing can perceive pulse-modulated fields with fre-
cat brain cells, but not with cultured rat nerve cells, chick
quencies between about 200 MHz and 6.5 GHz. The
skeletal muscle, or rat brain cells (Postow and Swicord
auditory sensation has been variously described as a
1996).
buzzing, clicking, or popping sound, depending on the
Amplitude-modulated fields have also been reported
modulation characteristics of the field. The microwave
to alter brain electrical activity (Bawin et al. 1974),
hearing effects have been attributed to a thermoelastic
inhibit T-lymphocyte cytotoxic activity (Lyle et al.
interaction in the auditory cortex of the brain, with a
1983), decrease the activities of non-cyclic-AMP-
threshold for perception of about 100 – 400 mJ m 2 for
dependent kinase in lymphocytes (Byus et al. 1984), and
pulses of duration less than 30
s at 2.45 GHz (corre-
cause a transient increase in the cytoplasmic activity of
sponding to an SA of 4 –16 mJ kg 1). Repeated or
ornithine decarboxylase, an essential enzyme for cell
prolonged exposure to microwave auditory effects may
proliferation (Byus et al. 1988; Litovitz et al. 1992). In
be stressful and potentially harmful.
contrast, no effects have been observed on a wide variety
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
507
● ICNIRP GUIDELINES
of other cellular systems and functional end-points,
Table 3. Ranges of threshold currents for indirect effects, includ-
including lymphocyte capping, neoplastic cell transfor-
ing children, women, and men.
mation, and various membrane electrical and enzymatic
Threshold current (mA) at
properties (Postow and Swicord 1996). Of particular
frequency:
relevance to the potential carcinogenic effects of pulsed
Indirect effect
100 kHz
1 MHz
fields is the observation by Balcer-Kubiczek and Harri-
son (1991) that neoplastic transformation was acceler-
Touch perception
25–40
25–40
Pain on finger contact
33–55
28–50
ated in C3H/10T1/2 cells exposed to 2,450-MHz micro-
Painful shock/let-go threshold
112–224
Not determined
waves that were pulse-modulated at 120 Hz. The effect
Severe shock/breathing difficulty
160–320
Not determined
was dependent on field strength but occurred only when
a chemical tumor-promoter, TPA, was present in the cell
culture medium. This finding suggests that pulsed micro-
waves may exert co-carcinogenic effects in combination
Summary of biological effects and epidemiological
with a chemical agent that increases the rate of prolifer-
studies (100 kHz–300 GHz)
ation of transformed cells. To date, there have been no
Available experimental evidence indicates that the
exposure of resting humans for approximately 30 min to
attempts to replicate this finding, and its implication for
EMF
producing
a
whole-body
SAR
of
between
human health effects is unclear.
1 and 4 W kg 1 results in a body temperature increase of
Interpretation of several observed biological effects
less than 1 °C. Animal data indicate a threshold for
of AM electromagnetic fields is further complicated by
behavioral responses in the same SAR range. Exposure
the apparent existence of “windows” of response in both
to more intense fields, producing SAR values in excess
the power density and frequency domains. There are no
of 4 W kg 1, can overwhelm the thermoregulatory
accepted models that adequately explain this phenome-
capacity of the body and produce harmful levels of tissue
non, which challenges the traditional concept of a mono-
heating. Many laboratory studies with rodent and non-
tonic relationship between the field intensity and the
human primate models have demonstrated the broad
severity of the resulting biological effects.
range of tissue damage resulting from either partial-body
Overall, the literature on athermal effects of AM
or whole-body heating producing temperature rises in
electromagnetic fields is so complex, the validity of
excess of 1–2°C. The sensitivity of various types of
reported effects so poorly established, and the relevance
tissue to thermal damage varies widely, but the threshold
of the effects to human health is so uncertain, that it is
for irreversible effects in even the most sensitive tissues
impossible to use this body of information as a basis for
is greater than 4 W kg 1 under normal environmental
setting limits on human exposure to these fields.
conditions. These data form the basis for an occupational
exposure restriction of 0.4 W kg 1, which provides a
large margin of safety for other limiting conditions such
Indirect effects of electromagnetic fields
as high ambient temperature, humidity, or level of
In the frequency range of about 100 kHz–110 MHz,
physical activity.
Both laboratory data and the results of limited
shocks and burns can result either from an individual
human studies (Michaelson and Elson 1996) make it
touching an ungrounded metal object that has acquired a
clear that thermally stressful environments and the use of
charge in a field or from contact between a charged
drugs or alcohol can compromise the thermoregulatory
individual and a grounded metal object. It should be
capacity of the body. Under these conditions, safety
noted that the upper frequency for contact current (110
factors should be introduced to provide adequate protec-
MHz) is imposed by a lack of data on higher frequencies
tion for exposed individuals.
rather than by the absence of effects. However, 110 MHz
Data on human responses to high-frequency EMF
is the upper frequency limit of the FM broadcast band.
that produce detectable heating have been obtained from
Threshold currents that result in biological effects rang-
controlled exposure of volunteers and from epidemiolog-
ing in severity from perception to pain have been
ical studies on workers exposed to sources such as radar,
measured in controlled experiments on volunteers (Chat-
medical diathermy equipment, and heat sealers. They are
terjee et al. 1986; Tenforde and Kaune 1987; Bernhardt
fully supportive of the conclusions drawn from labora-
1988); these are summarized in Table 3. In general, it has
tory work, that adverse biological effects can be caused
been shown that the threshold currents that produce
by temperature rises in tissue that exceed 1°C. Epidemi-
perception and pain vary little over the frequency range
ological studies on exposed workers and the general
100 kHz–1 MHz and are unlikely to vary significantly
public have shown no major health effects associated
over the frequency range up to about 110 MHz. As noted
with typical exposure environments. Although there are
earlier for lower frequencies, significant variations be-
deficiencies in the epidemiological work, such as poor
tween the sensitivities of men, women, and children also
exposure assessment, the studies have yielded no con-
exist for higher frequency fields. The data in Table 3
vincing evidence that typical exposure levels lead to
represent the range of 50th percentile values for people of
adverse reproductive outcomes or an increased cancer
different sizes and different levels of sensitivity to
risk in exposed individuals. This is consistent with the
contact currents.
results of laboratory research on cellular and animal
508
Health Physics
April 1998, Volume 74, Number 4
models, which have demonstrated neither teratogenic nor
are current density, SAR, and power density. Protection
carcinogenic effects of exposure to athermal levels of
against adverse health effects requires that these basic
high-frequency EMF.
restrictions are not exceeded.
Exposure to pulsed EMF of sufficient intensity leads
Reference levels of exposure are provided for com-
to certain predictable effects such as the microwave
parison with measured values of physical quantities;
hearing phenomenon and various behavioral responses.
compliance with all reference levels given in these guide-
Epidemiological studies on exposed workers and the
lines will ensure compliance with basic restrictions. If
general public have provided limited information and
measured values are higher than reference levels, it does not
failed to demonstrate any health effects. Reports of
necessarily follow that the basic restrictions have been
severe retinal damage have been challenged following
exceeded, but a more detailed analysis is necessary to assess
unsuccessful attempts to replicate the findings.
compliance with the basic restrictions.
A large number of studies of the biological effects of
amplitude-modulated EMF, mostly conducted with low
levels of exposure, have yielded both positive and neg-
General statement on safety factors
ative results. Thorough analysis of these studies reveals
There is insufficient information on the biological
that the effects of AM fields vary widely with the
and health effects of EMF exposure of human popula-
exposure parameters, the types of cells and tissues
tions and experimental animals to provide a rigorous
involved, and the biological end-points that are exam-
basis for establishing safety factors over the whole
ined. In general, the effects of exposure of biological
frequency range and for all frequency modulations. In
systems to athermal levels of amplitude-modulated EMF
addition, some of the uncertainty regarding the appropri-
are small and very difficult to relate to potential health
ate safety factor derives from a lack of knowledge
effects. There is no convincing evidence of frequency
regarding the appropriate dosimetry (Repacholi 1998).
and power density windows of response to these fields.
The following general variables were considered in the
Shocks and burns can be the adverse indirect effects
development of safety factors for high-frequency fields:
of high-frequency EMF involving human contact with
● effects of EMF exposure under severe environ-
metallic objects in the field. At frequencies of 100
mental conditions (high temperature, etc.) and/or
kHz–110 MHz (the upper limit of the FM broadcast
high activity levels; and
band), the threshold levels of contact current that produce
● the potentially higher thermal sensitivity in cer-
effects ranging from perception to severe pain do not
tain population groups, such as the frail and/or
vary significantly as a function of the field frequency.
elderly, infants and young children, and people
The threshold for perception ranges from 25 to 40 mA in
individuals of different sizes, and that for pain from
with diseases or taking medications that compro-
approximately 30 to 55 mA; above 50 mA there may be
mise thermal tolerance.
severe burns at the site of tissue contact with a metallic
The following additional factors were taken into ac-
conductor in the field.
count in deriving reference levels for high-frequency fields:
GUIDELINES FOR LIMITING EMF EXPOSURE
● differences in absorption of electromagnetic en-
ergy by individuals of different sizes and different
Occupational and general public exposure
orientations relative to the field; and
limitations
● reflection, focusing, and scattering of the incident
The occupationally exposed population consists of
field, which can result in enhanced localized
adults who are generally exposed under known condi-
absorption of high-frequency energy.
tions and are trained to be aware of potential risk and to
take appropriate precautions. By contrast, the general
Basic restrictions
public comprises individuals of all ages and of varying
Different scientific bases were used in the develop-
health status, and may include particularly susceptible
ment of basic exposure restrictions for various frequency
groups or individuals. In many cases, members of the
ranges:
public are unaware of their exposure to EMF. Moreover,
individual members of the public cannot reasonably be
● Between 1 Hz and 10 MHz, basic restrictions are
expected to take precautions to minimize or avoid expo-
provided on current density to prevent effects on
sure. It is these considerations that underlie the adoption
nervous system functions;
of more stringent exposure restrictions for the public than
● Between 100 kHz and 10 GHz, basic restrictions
for the occupationally exposed population.
on SAR are provided to prevent whole-body heat
stress and excessive localized tissue heating; in
Basic restrictions and reference levels
the 100 kHz–10 MHz range, restrictions are
Restrictions on the effects of exposure are based on
provided on both current density and SAR; and
established health effects and are termed basic restric-
● Between 10 and 300 GHz, basic restrictions are
tions. Depending on frequency, the physical quantities
provided on power density to prevent excessive
used to specify the basic restrictions on exposure to EMF
heating in tissue at or near the body surface.
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
509
● ICNIRP GUIDELINES
In the frequency range from a few Hz to 1 kHz, for
In the low-frequency range, there are currently few
levels of induced current density above 100 mA m 2, the
data relating transient currents to health effects. The
thresholds for acute changes in central nervous system
ICNIRP therefore recommends that the restrictions on
excitability and other acute effects such as reversal of the
current densities induced by transient or very short-term
visually evoked potential are exceeded. In view of the
peak fields be regarded as instantaneous values which
safety considerations above, it was decided that, for
should not be time-averaged.
frequencies in the range 4 Hz to 1 kHz, occupational
The basic restrictions for current densities, whole-
exposure should be limited to fields that induce current
body average SAR, and localized SAR for frequencies
densities less than 10 mA m 2, i.e., to use a safety factor
between 1 Hz and 10 GHz are presented in Table 4, and
of 10. For the general public an additional factor of 5 is
those for power densities for frequencies of 10 –300 GHz
applied, giving a basic exposure restriction of 2 mA m 2.
are presented in Table 5.
Below 4 Hz and above 1 kHz, the basic restriction on
induced current density increases progressively, corre-
REFERENCE LEVELS
sponding to the increase in the threshold for nerve
stimulation for these frequency ranges.
Where appropriate, the reference levels are obtained
Established biological and health effects in the
from the basic restrictions by mathematical modeling and
frequency range from 10 MHz to a few GHz are
by extrapolation from the results of laboratory investiga-
consistent with responses to a body temperature rise of
tions at specific frequencies. They are given for the condi-
more than 1°C. This level of temperature increase results
tion of maximum coupling of the field to the exposed
from exposure of individuals under moderate environ-
individual, thereby providing maximum protection. Tables
mental conditions to a whole-body SAR of approxi-
6 and 7 summarize the reference levels for occupational
mately 4 W kg 1 for about 30 min. A whole-body
exposure and exposure of the general public, respectively,
average SAR of 0.4 W kg 1 has therefore been chosen as
and the reference levels are illustrated in Figs. 1 and 2. The
the restriction that provides adequate protection for
reference levels are intended to be spatially averaged values
occupational exposure. An additional safety factor of 5 is
over the entire body of the exposed individual, but with the
introduced for exposure of the public, giving an average
important proviso that the basic restrictions on localized
whole-body SAR limit of 0.08 W kg 1.
exposure are not exceeded.
The lower basic restrictions for exposure of the
For low-frequency fields, several computational and
general public take into account the fact that their age and
measurement methods have been developed for deriving
health status may differ from those of workers.
field-strength reference levels from the basic restrictions.
Table 4. Basic restrictions for time varying electric and magnetic fields for frequencies up to 10 GHz.a
Current density for
Whole-body
Localized SAR
Exposure
head and trunk
average SAR
(head and trunk)
Localized SAR
characteristics
Frequency range
(mA m 2) (rms)
(W kg 1)
(W kg 1)
(limbs) (W kg 1)
Occupational
up to 1 Hz
40
—
—
—
exposure
1–4 Hz
40/f
—
—
—
4 Hz–1 kHz
10
—
—
—
1–100 kHz
f/100
—
—
—
100 kHz–10 MHz
f/100
0.4
10
20
10 MHz–10 GHz
—
0.4
10
20
General public
up to 1 Hz
8
—
—
—
exposure
1–4 Hz
8/f
—
—
—
4 Hz–1 kHz
2
—
—
—
1–100 kHz
f/500
—
—
—
100 kHz–10 MHz
f/500
0.08
2
4
10 MHz–10 GHz
—
0.08
2
4
a Note:
1. f is the frequency in hertz.
2. Because of electrical inhomogeneity of the body, current densities should be averaged over a cross-section of 1 cm2 perpendicular
to the current direction.
3. For frequencies up to 100 kHz, peak current density values can be obtained by multiplying the rms value by 2 ( 1.414). For pulses
of duration t the equivalent frequency to apply in the basic restrictions should be calculated as f
1/(2t ).
p
p
4. For frequencies up to 100 kHz and for pulsed magnetic fields, the maximum current density associated with the pulses can be
calculated from the rise/fall times and the maximum rate of change of magnetic flux density. The induced current density can then
be compared with the appropriate basic restriction.
5. All SAR values are to be averaged over any 6-min period.
6. Localized SAR averaging mass is any 10 g of contiguous tissue; the maximum SAR so obtained should be the value used for the
estimation of exposure.
7. For pulses of duration t the equivalent frequency to apply in the basic restrictions should be calculated as f
1/(2t ). Additionally,
p
p
for pulsed exposures in the frequency range 0.3 to 10 GHz and for localized exposure of the head, in order to limit or avoid auditory
effects caused by thermoelastic expansion, an additional basic restriction is recommended. This is that the SA should not exceed
10 mJ kg 1 for workers and 2mJ kg 1 for the general public, averaged over 10 g tissue.
510
Health Physics
April 1998, Volume 74, Number 4
Table 5. Basic restrictions for power density for frequencies
The induced current density distribution varies inversely
between 10 and 300 GHz.a
with the body cross-section and may be relatively high in
Exposure characteristics
Power density (W m 2)
the neck and ankles. The exposure level of 5 kV m 1 for
exposure of the general public corresponds, under worst-
Occupational exposure
50
case conditions, to an induced current density of about 2
General public
10
mA m 2 in the neck and trunk of the body if the E-field
a Note:
vector is parallel to the body axis (ILO 1994; CRP 1997).
1. Power densities are to be averaged over any 20 cm2 of exposed area and
However, the current density induced by 5 kV m 1 will
any 68/f 1.05-min period (where f is in GHz) to compensate for
progressively shorter penetration depth as the frequency increases.
comply with the basic restrictions under realistic worst-
2. Spatial maximum power densities, averaged over 1 cm2, should not
case exposure conditions.
exceed 20 times the values above.
For purposes of demonstrating compliance with the
basic restrictions, the reference levels for the electric and
magnetic fields should be considered separately and not
additively. This is because, for protection purposes, the
The simplifications that have been used to date did not
currents induced by electric and magnetic fields are not
account for phenomena such as the inhomogeneous distri-
additive.
bution and anisotropy of the electrical conductivity and
For the specific case of occupational exposures at
other tissue factors of importance for these calculations.
frequencies up to 100 kHz, the derived electric fields can
The frequency dependence of the reference field
be increased by a factor of 2 under conditions in which
levels is consistent with data on both biological effects
adverse indirect effects from contact with electrically
and coupling of the field.
charged conductors can be excluded.
Magnetic field models assume that the body has a
At frequencies above 10 MHz, the derived electric
homogeneous and isotropic conductivity and apply sim-
and magnetic field strengths were obtained from the
ple circular conductive loop models to estimate induced
whole-body SAR basic restriction using computational
currents in different organs and body regions, e.g., the
and experimental data. In the worst case, the energy
head, by using the following equation for a pure sinusoi-
coupling reaches a maximum between 20 MHz and
dal field at frequency f derived from Faraday’s law of
several hundred MHz. In this frequency range, the
induction:
derived reference levels have minimum values. The
J
R f B,
(4)
derived magnetic field strengths were calculated from the
electric field strengths by using the far-field relationship
where B is the magnetic flux density and R is the radius
between E and H (E/H
377 ohms). In the near-field,
of the loop for induction of the current. More complex
the SAR frequency dependence curves are no longer
models use an ellipsoidal model to represent the trunk or
valid; moreover, the contributions of the electric and
the whole body for estimating induced current densities
magnetic field components have to be considered sepa-
at the surface of the body (Reilly 1989, 1992).
rately. For a conservative approximation, field exposure
If, for simplicity, a homogeneous conductivity of
levels can be used for near-field assessment since the
0.2 S m 1 is assumed, a 50-Hz magnetic flux density of
coupling of energy from the electric or magnetic field
100
T
generates
current
densities
between
contribution cannot exceed the SAR restrictions. For a
0.2 and 2 mA m 2 in the peripheral area of the body
less conservative assessment, basic restrictions on the
(CRP 1997). According to another analysis (NAS 1996),
whole-body average and local SAR should be used.
60-Hz exposure levels of 100
T correspond to average
Reference levels for exposure of the general public
current densities of 0.28 mA m 2 and to maximum
have been obtained from those for occupational exposure
current densities of approximately 2 mA m 2. More
by using various factors over the entire frequency range.
realistic calculations based on anatomically and electri-
These factors have been chosen on the basis of effects
cally refined models (Xi and Stuchly 1994) resulted in
that are recognized as specific and relevant for the
maximum current densities exceeding 2 mA m 2 for a
various frequency ranges. Generally speaking, the factors
100- T field at 60 Hz. However, the presence of biolog-
follow the basic restrictions over the entire frequency
ical cells affects the spatial pattern of induced currents
range, and their values correspond to the mathematical
and fields, resulting in significant differences in both
relation between the quantities of the basic restrictions
magnitude (a factor of 2 greater) and patterns of flow of
and the derived levels as described below:
the induced current compared with those predicted by
simplified analyses (Stuchly and Xi 1994).
● In the frequency range up to 1 kHz, the general
Electric field models must take into account the fact
public reference levels for electric fields are
that, depending on the exposure conditions and the size,
one-half of the values set for occupational expo-
shape, and position of the exposed body in the field, the
sure. The value of 10 kV m 1 for a 50-Hz or 8.3
surface charge density can vary greatly, resulting in a
kV m 1 for a 60-Hz occupational exposure in-
variable and non-uniform distribution of currents inside
cludes a sufficient safety margin to prevent stim-
the body. For sinusoidal electric fields at frequencies
ulation effects from contact currents under all
below about 10 MHz, the magnitude of the induced
possible conditions. Half of this value was cho-
current density inside the body increases with frequency.
sen for the general public reference levels, i.e.,
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
511
● ICNIRP GUIDELINES
Table 6. Reference levels for occupational exposure to time-varying electric and magnetic fields (unperturbed rms
values).a
E-field strength
H-field strength
B-field
Equivalent plane wave
Frequency range
(V m 1)
(A m 1)
( T)
power density S
(W m 2)
eq
up to 1 Hz
—
1.63
105
2
105
—
1–8 Hz
20,000
1.63
105/f 2
2
105/f 2
—
8–25 Hz
20,000
2
104/f
2.5
104/f
—
0.025–0.82 kHz
500/f
20/f
25/f
—
0.82–65 kHz
610
24.4
30.7
—
0.065–1 MHz
610
1.6/f
2.0/f
—
1–10 MHz
610/f
1.6/f
2.0/f
—
10–400 MHz
61
0.16
0.2
10
400–2,000 MHz
3f 1/2
0.008f 1/2
0.01f 1/2
f/40
2–300 GHz
137
0.36
0.45
50
a Note:
1. f as indicated in the frequency range column.
2. Provided that basic restrictions are met and adverse indirect effects can be excluded, field strength values can be exceeded.
3. For frequencies between 100 kHz and 10 GHz, S , E2, H2, and B2 are to be averaged over any 6-min period.
eq
4. For peak values at frequencies up to 100 kHz see Table 4, note 3.
5. For peak values at frequencies exceeding 100 kHz see Figs. 1 and 2. Between 100 kHz and 10 MHz, peak values for the field
strengths are obtained by interpolation from the 1.5-fold peak at 100 kHz to the 32-fold peak at 10 MHz. For frequencies exceeding
10 MHz it is suggested that the peak equivalent plane wave power density, as averaged over the pulse width, does not exceed 1,000
times the S
restrictions, or that the field strength does not exceed 32 times the field strength exposure levels given in the table.
eq
6. For frequencies exceeding 10 GHz, S , E2, H2, and B2 are to be averaged over any 68/f 1.05-min period (f in GHz).
eq
7. No E-field value is provided for frequencies
1 Hz, which are effectively static electric fields. Electric shock from low impedance
sources is prevented by established electrical safety procedures for such equipment.
Table 7. Reference levels for general public exposure to time-varying electric and magnetic fields (unperturbed rms
values).a
E-field strength
H-field strength
B-field
Equivalent plane wave
Frequency range
(V m 1)
(A m 1)
( T)
power density S
(W m 2)
eq
up to 1 Hz
—
3.2
104
4
104
—
1–8 Hz
10,000
3.2
104/f 2
4
104/f 2
—
8–25 Hz
10,000
4,000/f
5,000/f
—
0.025–0.8 kHz
250/f
4/f
5/f
—
0.8–3 kHz
250/f
5
6.25
—
3–150 kHz
87
5
6.25
—
0.15–1 MHz
87
0.73/f
0.92/f
—
1–10 MHz
87/f 1/2
0.73/f
0.92/f
—
10–400 MHz
28
0.073
0.092
2
400–2,000 MHz
1.375f 1/2
0.0037f 1/2
0.0046f 1/2
f/200
2–300 GHz
61
0.16
0.20
10
a Note:
1. f as indicated in the frequency range column.
2. Provided that basic restrictions are met and adverse indirect effects can be excluded, field strength values can be exceeded.
3. For frequencies between 100 kHz and 10 GHz, S , E2, H2, and B2 are to averaged over any 6-min period.
eq
4. For peak values at frequencies up to 100 kHz see Table 4, note 3.
5. For peak values at frequencies exceeding 100 kHz see Figs. 1 and 2. Between 100 kHz and 10 MHz, peak values for the field
strengths are obtained by interpolation from the 1.5-fold peak at 100 kHz to the 32-fold peak at 10 MHz. For frequencies exceeding
10 MHz it is suggested that the peak equivalent plane wave power density, as averaged over the pulse width does not exceed 1,000
times the S
restrictions, or that the field strength does not exceed 32 times the field strength exposure levels given in the table.
eq
6. For frequencies exceeding 10 GHz, S , E2, H2, and B2 are to be averaged over any 68/f 1.05-min period (f in GHz).
eq
7. No E-field value is provided for frequencies
1 Hz, which are effectively static electric fields. perception of surface electric charges
will not occur at field strengths less than 25 kVm 1. Spark discharges causing stress or annoyance should be avoided.
5 kV m 1 for 50 Hz or 4.2 kV m 1 for 60 Hz, to
● In the frequency range 100 kHz–10 MHz, the
prevent adverse indirect effects for more than
general public reference levels for magnetic
90% of exposed individuals;
fields have been increased compared with the
● In the low-frequency range up to 100 kHz, the
limits given in the 1988 IRPA guideline. In that
general public reference levels for magnetic fields
guideline, the magnetic field strength reference
are set at a factor of 5 below the values set for
levels were calculated from the electric field
occupational exposure;
strength reference levels by using the far-field
512
Health Physics
April 1998, Volume 74, Number 4
Fig. 1. Reference levels for exposure to time varying electric fields (compare Tables 6 and 7).
Fig. 2. Reference levels for exposure to time varying magnetic fields (compare Tables 6 and 7).
formula relating E and H. These reference
● In the high-frequency range 10 MHz–10 GHz, the
levels are too conservative, since the magnetic
general public reference levels for electric and
field at frequencies below 10 MHz does not
magnetic fields are lower by a factor of 2.2 than
contribute significantly to the risk of shocks,
those set for occupational exposure. The factor of
burns, or surface charge effects that form a
2.2 corresponds to the square root of 5, which is
major basis for limiting occupational exposure
the safety factor between the basic restrictions for
to electric fields in that frequency range;
occupational exposure and those for general public
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
513
● ICNIRP GUIDELINES
exposure. The square root is used to relate the
threshold contact currents that elicit biological responses
quantities “field strength” and “power density;”
in children and adult women are approximately one-half
● In the high-frequency range 10 –300 GHz, the
and two-thirds, respectively, of those for adult men, the
general public reference levels are defined by the
reference levels for contact current for the general public
power density, as in the basic restrictions, and are
are set lower by a factor of 2 than the values for
lower by a factor of 5 than the occupational
occupational exposure.
exposure restrictions;
For the frequency range 10 –110 MHz, reference
● Although little information is available on the
levels are provided for limb currents that are below the
relation between biological effects and peak val-
basic restrictions on localized SAR (see Table 9).
ues of pulsed fields, it is suggested that, for
frequencies exceeding 10 MHz, S
as averaged
SIMULTANEOUS EXPOSURE TO MULTIPLE
eq
over the pulse width should not exceed 1,000
FREQUENCY FIELDS
times the reference levels or that field strengths
It is important to determine whether, in situations of
should not exceed 32 times the field strength
simultaneous exposure to fields of different frequencies,
reference levels given in Tables 6 and 7 or shown
these exposures are additive in their effects. Additivity
in Figs. 1 and 2. For frequencies between about
should be examined separately for the effects of thermal
0.3 GHz and several GHz, and for localized
and electrical stimulation, and the basic restrictions
exposure of the head, in order to limit or avoid
below should be met. The formulae below apply to
auditory effects caused by thermoelastic expan-
relevant frequencies under practical exposure situations.
sion the specific absorption from pulses must
For electrical stimulation, relevant for frequencies
limited. In this frequency range, the threshold SA
up to 10 MHz, induced current densities should be added
of 4 –16 mJ kg 1 for producing this effect corre-
according to
sponds, for 30- s pulses, to peak SAR values of
130 –520 W kg 1 in the brain. Between 100 kHz
10 MHz
Ji
and 10 MHz, peak values for the field strengths in
1.
(5)
J
Figs. 1 and 2 are obtained by interpolation from
L, i
i
1 Hz
the 1.5-fold peak at 100 kHz to the 32-fold peak
For thermal effects, relevant above 100 kHz, SAR
at 10 MHz.
and power density values should be added according to:
● In Tables 6 and 7, as well as in Figs. 1 and 2,
different frequency break-points occur for occu-
10 GHz
SAR
300 GHz
i
Si
pational and general public derived reference
1,
(6)
levels. This is a consequence of the varying
SARL
SL
i
100 kHz
i
10 GHz
factors used to derive the general public reference
where
levels, while generally keeping the frequency
dependence the same for both occupational and
J
the current density induced at frequency i;
i
general public levels.
J
the induced current density restriction at
L, i
frequency i as given in Table 4;
SAR
the SAR caused by exposure at frequency i;
REFERENCE LEVELS FOR CONTACT AND
i
SAR
the SAR limit given in Table 4;
INDUCED CURRENTS
L
S
the power density limit given in Table 5;
L
Up to 110 MHz, which includes the FM radio
and
transmission frequency band, reference levels for contact
S
the power density at frequency i.
i
current are given above which caution must be exercised
For practical application of the basic restrictions, the
to avoid shock and burn hazards. The point contact
following criteria regarding reference levels of field
reference levels are presented in Table 8. Since the
strengths should be applied.
Table 8. Reference levels for time varying contact currents from
Table 9. Reference levels for current induced in any limb at
conductive objects.a
frequencies between 10 and 110 MHz.a
Maximum contact
Exposure characteristics
Current (mA)
Exposure characteristics
Frequency range
current (mA)
Occupational exposure
100
Occupational exposure
up to 2.5 kHz
1.0
General public
45
2.5–100 kHz
0.4f
a
100 kHz–110 MHz
40
Note:
General public exposure
up to 2.5 kHz
0.5
1. The public reference level is equal to the occupational reference level
2.5–100 kHz
0.2f
divided by
5.
100 kHz–110 MHz
20
2. For compliance with the basic restriction on localized SAR, the square
root of the time-averaged value of the square of the induced current over
a f is the frequency in kHz.
any 6-min period forms the basis of the reference levels.
514
Health Physics
April 1998, Volume 74, Number 4
For induced current density and electrical stimula-
For limb current and contact current, respectively,
tion effects, relevant up to 10 MHz, the following two
the following requirements should be applied:
requirements should be applied to the field levels:
110 MHz
I
2
110 MHz
k
In
1 MHz
E
10 MHz
1
1,
(11)
i
Ei
I
I
1,
(7)
L, k
C, n
k
10 MHz
n
1 Hz
EL,i
a
i
1 Hz
i
1 MHz
where
and
I
the limb current component at frequency k;
k
I
the reference level of limb current (see Table
65 kHz
H
10 MHz
L, k
j
Hj
9);
1,
(8)
H
I
the contact current component at frequency
L, j
b
n
j
1 Hz
j
65 kHz
n; and
where
I
the reference level of contact current at
C, n
frequency n (see Table 8).
E
the electric field strength at frequency i;
i
E
the electric field reference level from Tables
L, i
The above summation formulae assume worst-case
6 and 7;
conditions among the fields from the multiple sources.
H
the magnetic field strength at frequency j;
j
As a result, typical exposure situations may in practice
H
the magnetic field reference level from Ta-
L, j
require less restrictive exposure levels than indicated by
bles 6 and 7;
the above formulae for the reference levels.
a
610 V m 1 for occupational exposure and
87 V m 1 for general public exposure; and
b
24.4 A m 1 (30.7
T) for occupational expo-
PROTECTIVE MEASURES
sure and 5 A m 1 (6.25 T) for general public
exposure.
ICNIRP notes that the industries causing exposure
to electric and magnetic fields are responsible for ensur-
The constant values a and b are used above 1 MHz
ing compliance with all aspects of the guidelines.
for the electric field and above 65 kHz for the magnetic
Measures for the protection of workers include
field because the summation is based on induced current
engineering and administrative controls, personal protec-
densities and should not be mixed with thermal consider-
tion programs, and medical surveillance (ILO 1994).
ations. The latter forms the basis for E
and H
above 1
Appropriate protective measures must be implemented
L,i
L,j
MHz and 65 kHz, respectively, found in Tables 6 and 7.
when exposure in the workplace results in the basic
For thermal considerations, relevant above 100 kHz,
restrictions being exceeded. As a first step, engineering
the following two requirements should be applied to the
controls should be undertaken wherever possible to
field levels:
reduce device emissions of fields to acceptable levels.
Such controls include good safety design and, where
1 MHz
E 2
300 GHz
2
necessary, the use of interlocks or similar health protec-
i
Ei
1,
(9)
tion mechanisms.
c
EL,i
Administrative controls, such as limitations on ac-
i
100 kHz
i
1 MHz
cess and the use of audible and visible warnings, should
and
be used in conjunction with engineering controls. Per-
sonal protection measures, such as protective clothing,
1 MHz
H 2
300 GHz
2
j
Hj
though useful in certain circumstances, should be re-
1,
(10)
d
H
garded as a last resort to ensure the safety of the worker;
L, j
j
100 kHz
j
1 MHz
priority should be given to engineering and administra-
where
tive controls wherever possible. Furthermore, when such
items as insulated gloves are used to protect individuals
E
the electric field strength at frequency i;
i
from high-frequency shock and burns, the basic restric-
E
the electric field reference level from Tables
L, i
tions must not be exceeded, since the insulation protects
6 and 7;
only against indirect effects of the fields.
H
the magnetic field strength at frequency j;
j
With the exception of protective clothing and other
H
the magnetic field reference level from Ta-
L, i
personal protection, the same measures can be applied to
bles 6 and 7;
the general public whenever there is a possibility that the
c
610/f V m 1 (f in MHz) for occupational
general public reference levels might be exceeded. It is
exposure and 87/f 1/2 V m 1 for general
also essential to establish and implement rules that will
public exposure; and
prevent:
d
1.6/f A m 1 (f in MHz) for occupational
exposure and 0.73/f for general public expo-
● interference with medical electronic equipment
sure.
and devices (including cardiac pacemakers);
Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields
515
● ICNIRP GUIDELINES
● detonation of electro-explosive devices (detona-
Beall, C.; Delzell, E.; Cole, P.; Brill, I. Brain tumors among
tors); and
electronics industry workers. Epidemiology 7:125–130;
● fires and explosions resulting from ignition of
1996.
flammable materials by sparks caused by induced
Beniashvili, D. S.; Bilanishvili, V. G.; Menabde, M. Z. The
effect of low-frequency electromagnetic fields on the devel-
fields, contact currents, or spark discharges.
opment of experimental mammary tumors. Vopr. Onkol.
37:937–941; 1991.
Bergqvist, U. Pregnancy outcome and VDU work—a review.
Acknowledgments—The support received by ICNIRP from the Interna-
tional Radiation Protection Association, the World Health Organization,
In: Luczak, H.; Cakir, A.; An Cakir, G., eds. Work with
the United Nations Environment Programme, the International Labour
display units ‘92—Selected Proceedings of the 3rd Interna-
Office, the European Commission, and the German Government is grate-
tional Conference WWDO ‘92, Berlin Germany, 1– 4 Sep-
fully acknowledged.
tember 1992. Amsterdam: Elsevier; 1993: 70 –76.
Bernhardt, J. H. The direct influence of electromagnetic fields
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APPENDIX
Glossary
Dielectric constant. See permittivity.
Absorption. In radio wave propagation, attenuation
Dosimetry. Measurement, or determination by cal-
of a radio wave due to dissipation of its energy, i.e.,
culation, of internal electric field strength or induced
conversion of its energy into another form, such as heat.
current density, of the specific energy absorption, or
specific energy absorption rate distribution, in humans or
Athermal effect. Any effect of electromagnetic
animals exposed to electromagnetic fields.
energy on a body that is not a heat-related effect.
Electric field strength. The force (E) on a station-
Blood-brain barrier. A functional concept devel-
ary unit positive charge at a point in an electric field;
oped to explain why many substances that are trans-
measured in volt per meter (V m 1).
ported by blood readily enter other tissues but do not
enter the brain; the “barrier” functions as if it were a
Electromagnetic energy. The energy stored in an
continuous membrane lining the vasculature of the brain.
electromagnetic field. Expressed in joule (J).
These brain capillary endothelial cells form a nearly
continuous barrier to entry of substances into the brain
from the vasculature.
ELF. Extremely low frequency; frequency below
300 Hz.
Conductance. The reciprocal of resistance. Ex-
pressed in siemens (S).
EMF. Electric, magnetic, and electromagnetic
fields.
Conductivity, electrical. The scalar or vector quan-
tity which, when multiplied by the electric field strength,
Far field. The region where the distance from a
yields the conduction current density; it is the reciprocal
radiating antenna exceeds the wavelength of the radiated
of resistivity. Expressed in siemens per meter (S m 1).
EMF; in the far-field, field components (E and H) and the
direction of propagation are mutually perpendicular, and
Continuous wave. A wave whose successive oscil-
the shape of the field pattern is independent of the
lations are identical under steady-state conditions.
distance from the source at which it is taken.
Current density. A vector of which the integral
Frequency. The number of sinusoidal cycles com-
over a given surface is equal to the current flowing
pleted by electromagnetic waves in 1 s; usually ex-
through the surface; the mean density in a linear conduc-
pressed in hertz (Hz).
tor is equal to the current divided by the cross-sectional
area of the conductor. Expressed in ampere per square
Impedance, wave. The ratio of the complex number
meter (A m 2).
(vector) representing the transverse electric field at a
point to that representing the transverse magnetic field at
Depth of penetration. For a plane wave electro-
that point. Expressed in ohm ( ).
magnetic field (EMF), incident on the boundary of a
good conductor, depth of penetration of the wave is the
Magnetic field strength. An axial vector quantity,
depth at which the field strength of the wave has been
H, which, together with magnetic flux density, specifies
reduced to 1/e, or to approximately 37% of its original
a magnetic field at any point in space, and is expressed in
value.
ampere per meter (A m 1).
522
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April 1998, Volume 74, Number 4
Magnetic flux density. A vector field quantity, B,
magnetic field strength (multiplied by the impedance of
that results in a force that acts on a moving charge or
space) and the electric field strength are equal.
charges, and is expressed in tesla (T).
Power density. In radio wave propagation, the
Magnetic permeability. The scalar or vector quan-
power crossing a unit area normal to the direction of
tity which, when multiplied by the magnetic field
wave propagation; expressed in watt per square meter
strength, yields magnetic flux density; expressed in henry
(W m 2).
per meter (H m 1). Note: For isotropic media, magnetic
permeability is a scalar; for anisotropic media, it is a
Public exposure. All exposure to EMF experienced
tensor quantity.
by members of the general public, excluding occupa-
tional exposure and exposure during medical procedures.
Microwaves. Electromagnetic radiation of suffi-
ciently short wavelength for which practical use can be
Radiofrequency (RF). Any frequency at which
made of waveguide and associated cavity techniques in
electromagnetic radiation is useful for telecommunica-
its transmission and reception. Note: The term is taken to
tion. Note: In this publication, radiofrequency refers to
signify radiations or fields having a frequency range of
the frequency range 300 Hz –300 GHz.
300 MHz–300 GHz.
Resonance. The change in amplitude occurring as
Near field. The region where the distance from a
the frequency of the wave approaches or coincides with
radiating antenna is less than the wavelength of the
a natural frequency of the medium; whole-body absorp-
radiated EMF. Note: The magnetic field strength (multi-
tion of electromagnetic waves presents its highest value,
plied by the impedance of space) and the electric field
i.e., the resonance, for frequencies (in MHz) correspond-
strength are unequal and, at distances less than one-tenth
ing approximately to 114/L, where L is the height of the
of a wavelength from an antenna, vary inversely as the
individual in meters.
square or cube of the distance if the antenna is small
compared with this distance.
Root mean square (rms). Certain electrical effects
are proportional to the square root of the mean of the
Non-ionizing radiation (NIR). Includes all radia-
square of a periodic function (over one period). This
tions and fields of the electromagnetic spectrum that do
value is known as the effective, or root-mean-square
not normally have sufficient energy to produce ionization
(rms) value, since it is derived by first squaring the
in matter; characterized by energy per photon less than
function, determining the mean value of the squares
about 12 eV, wavelengths greater than 100 nm, and
obtained, and taking the square root of that mean value.
frequencies lower than 3
1015 Hz.
Specific energy absorption. The energy absorbed
Occupational exposure. All exposure to EMF ex-
per unit mass of biological tissue, (SA) expressed in joule
perienced by individuals in the course of performing
per kilogram (J kg 1); specific energy absorption is the
their work.
time integral of specific energy absorption rate.
Permittivity. A constant defining the influence of
Specific energy absorption rate (SAR). The rate at
an isotropic medium on the forces of attraction or
which energy is absorbed in body tissues, in watt per
repulsion between electrified bodies, and expressed in
kilogram (W kg 1); SAR is the dosimetric measure that
farad per metre (F m 1); relative permittivity is the
has been widely adopted at frequencies above about 100
permittivity of a material or medium divided by the
kHz.
permittivity of vacuum.
Wavelength. The distance between two successive
Plane wave. An electromagnetic wave in which the
points of a periodic wave in the direction of propagation,
electric and magnetic field vectors lie in a plane perpen-
at which the oscillation has the same phase.
dicular to the direction of wave propagation, and the
f f
Document Outline
- INTRODUCTION
- PURPOSE AND SCOPE
- QUANTITIES AND UNITS
- BASIS FOR LIMITING EXPOSURE
- COUPLING MECHANISMS BETWEEN FIELDS AND THE BODY
- INDIRECT COUPLING MECHANISMS
- BIOLOGICAL BASIS FOR LIMITING EXPOSURE (UP TO 100 KHz)
- Direct effects of electric and magnetic fields
- Indirect effects of electric and magnetic fields
- Summary
- BIOLOGICAL BASIS FOR LIMITING EXPOSURE (100-300 GHz)
- Direct effects of electromagnetic fields
- Indirect effects of electromagnetic fields
- Summary
- GUIDELINES FOR LIMITING EMF EXPOSURE
- Occupational and general public exposure limitations
- Basic restrictions and reference levels
- General statement on safety factors
- Basic restrictions
- REFERENCE LEVELS
- REFERENCE LEVELS FOR CONTACT AND INDUCED CURRENTS
- SIMULTANEOUS EXPOSURE TO MULTIPLE FREQUENCY FIELDS
- PROTECTIVE MEASURES
- REFERENCES
- APPENDIX
